There are only two ways you can paint upside down with any gravity feed gun. One is use a bag system inside the cup. The other is to figure out some way to invert the cup. Some gravity feed guns allow the cup to be swiveled to any position, some do not. The bag system is the easiest solution to this problem. With the internal bag system all air is sucked from the bag inside the paint cup, and as the paint is pulled from the bag it collapses.
edit: another method would be to have a remote cup arrangement.
Vince