That plastikote stuff actually works pretty well, if the surface is clean and smooth to start with. I've used it on a motorcycle seat, and it's very durable- after more than a year, only a few small cracks were visible, and that's on a flexible surface that was exposed to heat, weather, etc- a much harsher environment than your k/b's going to face.
As far as actually dyeing the plastic, you really can't- it has to be done when it's cast. You might be able to get some in the surface, by using something like methyl ethyl ketone to melt the plastic, but then the surface would be uneven, and I doubt you could get an even coloring, anyway.
If you really thoroughly clean the keys and apply the coats evenly, it should come out looking pretty good. Letting each coat dry and then lightly buffing them with very fine steel wool before applying the next coat will make them smoother and shinier- but it'd also be an awfully big time-sink. As long as you avoid industrial solvents, I'd expect the plastikote to last for several years, at least.
(Off topic, wtf is up with non-acetone nail polish remover? It's made with methyl acetate or ethyl acetate, both of which are nastier than acetone. I guess because they smell better than acetone, people figure it must be safer. Gah.)
