Hi all.

Basically, what I want to do is make a door that when passed through a sim would get undressed (for nudist homes, etc.)

I know it should be simple enough, but I have absolutely no idea how to do it.

If anyone knows please let me know? It'd be highly appreciated.

Exnem, Inge Jones has already made a door that does this. It is in the Adult Downloads section and is called "PayClothes" door or something similiar. It lets you select what outfit the Sim will change into when it goes through, and include the option for naked. It can be used for NPCs (or not if you don't want them to be affected) and can change your Sims back into normal clothes when they pass through in the opposite direction. Download the patch he has with it to make your Sims not notice their nudity and it works like a charm. I am using it for my nudist Greek House at the moment and it is great.
On another note, I would be interested in knowing how the door works (as far as BHAV modification or constants) if anyone wants to chime in that's reading this.