Ok, I was going insane yesterday trying to make a new skin for one of the "Jedi Robes" for the human male Jedi model; however, one of the textures was NOT showing up as double sided when it was supposed to, and try as I might I could not figure out why since it looked fine in model viewer.
Well, this morning I finaly solved it and since I know other people have had this issue as well, here is how I finaly solved it:
There is a file in assets01.pk3 that contains info on specific textures that have special functions (like being double sided). This file is called players.shader and you will have to include it with your model's pk3 (no worries, its only 80k or so). After extracting the file, you'll need to go through it to find the file name of the original skin for the part you're modeling. Copy that section right bellow itself, and swap any references to the old skin with references to your own new skin. That should do it.
I imagine that multiple versions of this file cancel each other out, so it might be impossible to have multiple new models from diferent pk3s that use this feature; you can of course with a little patience go through each and make sure to add all new elements into a new players.shader which should let everything work.
I just figured this out so I haven't done much elaborate testing... I'm just glad I finaly got my skin to work
If anyone is having trouble adding new reskinned parts to the sp/custom model let me know, I think I finaly have it all figured out minus the color options.