I think you are right, Kaz's strive for perfection is probably one of the main reasons we don't have a livery editor. Geometry Intersection (thanks @
Griffith500) is the better option for quality, but using it practically is more complicated, and I suspect that certain things need to be built in to the original model to make it feasible - for instance the livery engine would need to know not to intersect glass surfaces, grilles, head lamps etc.. it's not impossible at all. But I think it's a lot of work, and needs to be approached from the ground up.
It definitely needs an editor of some sort, it really would open a new dimension for the game. If we get a decent course creator, and we had a decent livery editor, I think maybe only 30% of my time in GT would be spent driving