Disassembling a GPU is pretty easy, there is no real reason to avoid doing that if you're going to paint it. Taping everything off in order to paint the heatsink exterior is going to be a pain in the ass, and even then there is no guarantee you tape everything off properly. Not to mention if you're using plastidip, that painting things that don't need to be painted can be a issue in its own. Disassemble the thing as far as you can (a lot of heatsinks and fans can be separated from the outer shroud itself)
And yes, plastidip is almost certainly the material to use for this mod.
I'm not sure about custom backplates, but if you want one then go for it.
