Hmmm what you're talking about would place a massive strain on the master server, since not only would LFS world have to send all the skins to a player who had just joined the race, but it would then have to check to make sure that each of them wasn't using a restricted file, then it would have to issue a command to the private server to dump that player out of the race. Then it would have to check again when they rejoined to make sure they weren't still being "naughty".
As for checking the RGB values of random pixels, you'd need to map that to ensure that it only checked pixels that were visible on the car and therefore not likely to be a part of the black mask common to the Master Skinnerz kits for instance. Since I've seen plenty of skins which are primarily one colour, it would be almost impossible for an automated pixel checking system to work with any degree of reliability.
At the end of the day I appreciate it's frustrating and annoying to see somebody else wearing your pride and joy, when you wanted to be unique and keep it to yourself, but there just isn't an easy or even viable way to prevent skin stealing, and the whole point of using .jpgs instead of .dds for skins is to make skinning as accesible as possible. The fact that the devs have gone out of their way to make skinning as easy as possible right from the earliest days of LFS is to my mind proof that they want to encourage players to make their own paintjobs, and aren't going to be too keen on anything which will cost them time to code and money to resource in terms of extra load on LFS world.
I'm sorry that my response appears so negative, I've been where you are, my response was merely to remove the skin in question from LFS world, so that the only option the offender had was to double the size of the already poor quality 512 skin they'd got from skins_x and upload it themselves in order to continue using it. That was evidently beyond the person in question since I saw him a few days later driving a white car, and seeing the message "skin_abdcdef.jpg not found on lfsworld" as they repeatedly went in out of the pits, presumably doing some head scratching.
These days when I sit down to make a skin I remind myself that there isn't really a way for me to guarantee nobody else will use it, and ultimately that is the answer.