I'd think that, if you kept it upside down til the paint was completely dried, there wouldn't be any chance of any paint fumes getting into the tank.
My mom used to be a photo retoucher and used a lot of spraypaints and such in the garage near my dad's motorcycle. He never kept it covered [his own stupid idea] the entire time my mom used spray paints in the garage. She was no more than 2 feet from the motorcycle, and not once did any spray paint get on it. She had to wear a mask for the fumes while using it, but once it was dried it was safe.
Basically, as long as the tank stays upside down for a couple days or so, you'd be fine. No chance of stuff getting into the tank.