Frankly, if that's what you want to do, I don't see a problem. My greatest concern that the paint might not adhere well, would start to flake, and look pretty awful. I've yet to see a paint that is sufficiently conductive to load a battery; even metal-filled paints and adhesives are sometimes surprisingly nonconductive. In my opinion, the old idea that putting a battery on a concrete floor discharges it is just not correct, and the explanations for it are largely ad hoc. I suspect the apparent discharge is really just a poorly charged battery sitting on a cold floor, which reduces its output, or some similar phenomenon. But when someone shows me some real data proving that the phenomenon occurs (and I don't mean anecdotal experience) I'll change my opinion.