reason why this isn't as possible is that the default weapon uses a texture atlas, which can't be expanded on.
from my limited understanding - a texture atlas is a big ol picture with all the weapon parts' textures on it, and the game reads this one file per weapon. this helps a ton with performance, but you can't just take that atlas and add more stuff to it, since there's no more room.
This is why we make more skins - but I'll ask the team if we can get some new skins with some parts for customization :)