Hmmm, sorry to say, but I doubt it.
I'm preety sure it's all part of the car model. I say this because I swapped my 787B body bits for the 787B Stealth model's body bits and the livery and all the stickers changed, even the Mazda one on the windshield was replaced by the one saying Gran Turismo. That with just the body code.
If there's sticker codes anywhere, I believe they are on the actual game code and not on the save file.
I'm not sure, though.