I think theres one big problem with having the developer regulate which user made liveries make it into the game, and that copyright infringement.
If a player wants to create race replicas, or custom paintjobs that feature copyrighted imagery, theres no way the developer would allow those liveries into the game.
It would affect race replicas, because those paintjobs feature company logos, most of which PD wont be licensed to use, all the different decals and logos needed to replicate an existing livery, might be impossible (or incredibly time consuming) for PD to allow into the game.
It would also affect fantasy liveries , take a spongebob, southpark, batman livery, the characters and associated imagery has to be licensed, IF PD or Sony are in control of the distribution of custom liveries, all of that would have to be taken into account.
I might be wrong, im no expert in copyright law, but i cant see PD or Sony wanting to having the final say on liveries, the most they would want is to be able to enforce TOS, so no pornography or discriminatory imagery.