On some of the military forums, I have heard solders talking about spray-painting their packs, so it must work reasonably well, but they would be painting camo ON, not painting over camo...
Just a light dusting of spray-paint is needed, you do NOT want to coat the fabric, you just want a powder of paint on it, so that it does not get stiff...You can always spray some touch-up on it every year or so, if you are picky about that sort of thing...