here's something I've been playing with.
I took a headmodel from urbanterror's sarah character and a photo of a supermodel. I added a new uvmap to the headmodel and warped it onto the photo, I then cloned some of that image back onto the original texture. This means I can modify models textures in any way I like without having to modify the game's md3 models.
This is quite useful for headmodels, but if I really wanted I could easily modify the mesh for the headmodel. Where it is really useful is for the legs model and the torso/arms model. I can't modify those unless I have the original blender file that created them, like those are available with openarena, so this technique would be really useful for creating new skins for urban terror or quake3.
If anyone has any requests for custom skins, give me a shout and I'll see what I can do.