Great, simple method, Frank.
The only way I can make it work the way you'd like is to feed the actual 'parts shader' into the merge, instead of the object parts. There were also lots of render bucket warnings with everything I've tried inside, too.
Still, I'm not convinced it's impossible, though... Try duplicating the entire parts shader, only, inside of the secondary one, you could give the textures you don't want to touch with colour variation = no opacity, make them invisible. Then, merge mode = colour/add...
Maybe.