This would be usefull for categorizing skins, eg. if you have skins for different clothes on your character.

Definitely would love this. In addition, it'd also be really cool to be able to choose which folder(s) the game looks for skins in, rather than just .minecraft/skins. That way, you can have all of your skins in one folder (with possible subfolders, as per this suggestion) instead of needing to copy and paste between separate modded instances.