I know there are a few programs out there that allow you to interactively paint onto a 3D object, in real-time/real-space, but they aren't cheap.
One example is Right Hemisphere's "Deep 3D" ($595)
Of course, I'm sure you're not going to pay even $100 (let alone $600) just to paint one model! I'm just letting you know in case you'd like to look further into this and see if you can find a freebie version (not that freebie stuff really works the best :p).
Looks like a site with some legit freeware, but I didn't browse it completely, so I don't know if there is a program (like above) there.