O.K. I have come to the place where the smart people are.
I have made a new Pet Dog Collar (with original mesh). This is not a replacement object, but a new collar that shows up in CAS (Pets version).
Well, the collar works, but it is invisible. In the past that often meant an unlinked mesh. I copied the mesh into the package and relinked it, but that hasn't helped.
This is an interesting item because it is not in the catalog, and only shows up in CAS and on the Dog. It also uses tsDesignModeEnabled with a lot of texture names (in the GMND). I added my new name (to both subsets), but in this package I didn't delete the other recolor names.
At present, I have this seperated into two seperate packages. I tried housing everything in one package, using both 0xFFFFFFFF and 0x1C050000 groups, and then tried seperate packages because I was worried that they were relocating the references differently. STill broken. SimPE seems certain that the scenegraph is correct, and textures it correctly (Paleoanth contributed the texture) when previewing the mesh. Using debug mode (Shift-N) in CAS shows that it is looking for the correct scenegraph name (ducollarsmoothwh_bonesblack). But I think that I have it linked incorrectly.
My apologies for the unpolished nature of the package... the name says black, but it is orange, etc.
<* Wes *>

The screenshot shows the new collar icon (invisible)... the green border is where CAS thinks that the collar is now on the Dog (you can remove it). For some reason, it shows up in the very first slot for the collars, regardless of where I set the sort value at.

I am so downloading this and playing with it. Thanks for all the work you have done. I couldn't have even gotten this far.

All I wanted was a hunter orange collar like my dog has. Sheesh.

Well, I am answering myself here.
The problem was that I was not hashing the names in the scenegraph file properly (should be like ##0x1c050000!ducollarsmoothwh_cres).
I am attaching the fixed package and some screenshots from in CAS as well as in the game.

<* Wes *>

Note: The thumbnail did not update, and I didn't delete the cache, so it is blank in the CAS panel. Also, there is some other aspect that I did wrong because I have a mesh that is too large for the dog selected. Also, for testing I just used one texture for both subsets, so the buckle and the collar are both Paleoanths nice glowy orange color.
Anyone is welcome to try their hand at making their own collar packages, although this is essentially a complete clone and can probably be made in a more efficient, smaller package.
But, it's a start.

Edit: I have removed the package file attachment... there is a better version available in Tutorials.

Wes-

Still showing up at the old Bones collar for me. I even redownloaded the file and replaced the one where I added the collar in the Mat Def file and it still shows up as the bones collar.

O.K.
Well, I made it with a single texture initially because that is one less pair of links to mess up on (since I had to make this from scratch).
And I hope you mistyped when you said BodyShop. I only tested it in the Create-a-Sim (make a dog) and then put the dog in the game to see it work (the broken invisible package crashes the game when you try to save the dog).
<* Wes *>

Now, you did remove the old test package from downloads, didn't you???

Absolutely! Double checked that. I will triple check though.

Well, I tested again, but this time I deleted the cache files. On the first run, I got a proper icon, but it was now where it should be (based on the sort value I used), at the end of the list. I forgot to take a screen picture, so I ran the game again and I had a blank icon, in the last position, but it rendered the collar on the dog in CAS jsut fine.

I used a larger dog this time and the collar fits just right.

I hope someone else can help by testing if this works on some other game than mine.

<* Wes *>

Yeah, when I originally extracted the files, I used the ones for the big dogs.

Tested it with BGS, ran it 3 times. It worked always for big dogs, didn't show up for small ones. The icon was always displayed correctly - for big dogs. Age changes didn't disturp the icon or collar mesh. When testing it with a poodle I noticed that it doesn't change it's shape to fit the neck/fur - see screenshots. Hopefully this helps you somehow

But I hunted down all the pieces in there myself, except your texture files.
I think there is more to this than what I have here in this package. For example, there is a complete scenegraph "duCollarSmoothMask" besides the "duCollarSmooth" (which in my package is renamed to "duCollarSmoothWH"). The rename was intentional, because I want to have the GMDC in the package so I can play around with it. The GMND has to be there in order to put the txmt name in it.
I am suspecting that the ____Mask mesh helps CAS auto-adjust the mesh to the dog size. I put this mesh on a really big dog and it was stretched to fit, but torn a little at the seams, indicating that CAS is adjusting it somewhat.
I know that when the dog is saved into the neighborhood, a copy of the collar mesh is made (and maybe the textures, too) and saved in the character file. You can see this by using SimPE Neighborhood browser, finding your pet, and opening the character file. Selecting GMDC will yield around a dozen or so, and looking through them you'll find one with the buckle and collar groups. It will have a name vastly different from the original gmdc name. Preview it and you'll see the collar.
<* Wes *>

Thanks for the testing... I appreciate the help. At least I know it CAN work, even if it is a little buggy yet.

When I said big dogs, I was talking about larger necked versus thinner necked versions of the big dogs. I realize I was not being a clear as I should have been.

The big dogs stuff starts with "du". I forget what it is for the smaller dogs, and there is only a "du" part in here. For smaller dogs and for cats there would need to be more files in the package for the other prefixes and ages.

Oh, and thanks for the poodle test/screenshot. I will add that to my list of things to test when I work on this.

<* wes *>

I am also glad to know that it is showing up for others, I just have done something weird then.

I didn't change the guid in this from the one in the broken package.
There was a broken one I posted or sent you, all in one package, that had the guid from the bonesblack collar in it, instead of this one. That package created a new icon entry, but showed the bones mesh consistently.
I suspect that particular package is still somewhere in EA Games\The Sims 2.
I have found that the game will find stuff in about any directory you make in The Sims 2. When I want to remove a package temporarily I put it in a directory I made called "Ea Games\Moved", so I can find it to restore it later. When I had it as "EA Games\The Sims 2\Moved" it would show up anyway.
Before I tested it in downloads, I had it placed in collections and the game found it just the same.
<* Wes *>

Yes, non native speakers struggle with the meaning sometimes - I didn't think of the XXL meaning at all.
Didn't look at the packages in SimPE yet - busy translating some other packages - but it's another useful info from you

I think testing several shapes of fur will be required, for different heights, sizes and later on the same with cats too so you would have thought of it on your own, wouldn't you?

I realize that, and I one of my objectives has been to try to be more precise in my postings, at the expense of sounding a bit formal or of "talking down to" native (American) english speakers.

Personally, as much trouble as I have had learning a foreign language in my life (I found my way around Buenos Aires for a while, but I would never say I speak spanish, I just sort of faked it), I can imagine how difficult it is to learn english idioms, grammar and tenses (and the irregular ones), and then to learn all the technical verbs and nouns well enough to make meaningful posts here, as you do.

<* Wes *>

I myself prefer caling them "additional languages" (we could now start a philosophical discussion about belonging to the same species which is more intresting because of this mixture of differences and similarities which alltogether offer fascinating possibilities - and unfortunately lots of risky misunderstadnings and troubles, too - because we are all just humans. Though this would go beyond the scope of the topic. :D)

Speaking formal makes it easier to understand without seeming like someone talks down to non native English speakers. Slang is just harder to understand, although I want to learn as much as possible.

Thank you, nevertheless I just realized today that I too often just forget several things I want to say just because I'm too busy with trying to translate my thoughs Often enough I just feel helpless because I'm not able to express myself as well as I could in German - I just don't know the English phrases or correct words for it. Also getting the meaning is difficult sometimes so I'm sometimes just frustrated by my own limitations - I suppose you know what I mean as you should have experienced compareable situations in Buenos Aires

This was completely OT - if you have some more packages to test I'll be glad to help

Yeah, you think that's bad? I am sooo new to meshing, 5 months, tendonitis, red eyes, and a new monitor later. Heck all I wanted was to retexture lovely foufou collars for my doggies, and well, you see where it took us. On the upside, this has been yet another valuable classroom session for me.