If you're just talking about texture swaps, that's easy enough to do, but maintaining a "standard" can be problematic if multiple independent people decide to all create their own packs of texture edits (I've seen it before in another project, ended up causing a lot of confusion for people...).
If you're talking outright model edits/replacement, that might be trickier by a large measure... In order to answer that question, I have to ask another. Has anyone even figured out how to generate the proper file format for custom models to work in the game...? I've looked myself before and never found anything about how to generate/export into the correct file format, otherwise I'd have already been making new costume parts by now...