A mesh body with appliers and clothes with their HUD, from technical point of view, are the same thing.
You have to create a script to put inside your clothes: this script should listen ad handle "commands" from a HUD (with another script). The Hud have to send the UUID of the texture(s) you want to apply to clothes: the script inside the clothes will apply it to the mesh.
You can create the texture with any kind of paint software: tga (1024x1024) is the best texture format.
The texture have to use the right UV map.