Makes sense. There's nothing to stop the balls from just bouncing off the auger.
And if a cylinder surrounding the auger was put into place... well, then you have a Q loader, and it would have HORRIBLE feed rates due to only being open to paint at one end.
Skibbo, here's a quick mock up of what I had in mind based upon your design. Excuse the paint, I don't have a circuit design program at home.
Click Me!
In all honesty, I don't think this is the best way to go as you're limiting the current applied to the motor. You'd want the motor to be a completely different circuit, controlled via a relay.. but again, now we're diverging from the KISS method.