The hybrid cars rely on the reshuffling of data already found in the game. While you can hack paint some cars with livery (basically you swap out the body colour underneath the livery), to create a new livery would mean you have to add data to the game, (and we know how much trouble that can cause, look at the issues thrown up by each update). The resulting designs would be unreadable to any other copy of the game (so wouldn't be visible online, or could even crash other PS3's trying to read data that doesn't exist).
That said, if somebody did get it to work offline, there's nothing to stop someone adding new cars to the game, as is commonplace with PC games.
In theory if you could decompile an older GT game (back to GT3) you could copy the data of any car and repackage it in the same way PD has with standards. I think GT1 and 2 cars could be imported too with even more work (I'm almost certain that some GT3 models were no more than tarted up GT2 ones, GT One Road ver. Especially).
Of course this would all take a lot of knowledge and skill and really wouldn't be worth the bother.