It seems to be some type of plastic, I agree that a little sanding would be necessary.
I'm replacing the standard back with the leather one as soon as I receive it (est 29th). So if the painting doesn't work out, I don't really care, it will spend the next few years in a drawer with all the other misfit accessories from past devices otherwise (man I need to clean out that drawer).
This rubberized paint is just one idea, it may be too thick for this. I'm going to be on the lookout for other types of paint that might work also. I just figured with the premium back on the way, I might try something different with this one, it will never get used otherwise.