Might as well just use VHT nightshades and black them out that way. Plastidip would be difficult to spray just right. Think most states have it so tail lights must be visible from 300+ feet away. That's the main thing, if it's visible at those distances you should be fine.
i think plastidip is a bad idea. in order to peel it off without too much difficulty you have to spray it thick so i cant imagine light passing good enough. even if it does pass through well enough, would it "bake" on to your lights? vinyl is sexy and cheap and easily removable.
The states where I have looked it up usually state that tail lights need to be visible from 1000 feet away. As mentioned, this does not mean you won't be pulled over and given warnings if police don't like it.
According to me, you must refer convention collective metallurgie agreement for your area. There is well defined laws which is different for different region and car issues comes under metallurgy department. Selling your own car parts is legal.