This could be solved by a simple livery editor ala Forza. There was a lot of talk about this becoming a part of GT in the future so maybe one day it will. It would made Photomode much better for me. I always disliked the plates in the game, always made them look like show room models as opposed to real cars.
Still, this is really a very minor niggle and other things should be done to improve the game further first.