[Russell Sobie]

For changing specular on a texture, I am only just now dipping into that type of thing. I'm not sure all textures in the game actually have specular maps (or anything other than diffuse and in some cases light maps), and if they don't, I don't know if you can make one and get it referenced by the game. I'm about to start a new Audi skin that has silver paint, so I'll see if I can pull that off and report back. Helmets might be a bit more limited though. Not sure.

As far as painting anything on any of the car models, there has to be part of the existing UV map that will allow details at all. In the case of most cars, the windshield is mapped to a tiny 16x16 or something texture with an alpha channel that simply tiles. The cars that are exceptions are the ones that have the windshield number as part of the skin setup, like the Audi TT Cup. There's a specific texture that is mapped there which allows for details to be added.

I imagine if you wanted to do something like add a bit of mesh to the model of the car, you'd have to do some serious shenanigans to replace the actual model... and I don't think you'd be able to pull that off in under 10MB of data.