What about using 2 plexi plates of some sort? you could make a U slot on either side to let the plates ride on, and then your servo should be strong enough to pull the plexi out of the way. They don't have to be full size, just big enough to not let a ball go though.
Our robot uses a conveyors to pick up balls from the base of the hopper and bring them up to the shooter. Since our shooter design spins up incredibly fast, we simply have the shooter and conveyor tied to the same trigger on the joystick. i think we have less than half a second spin up on our shooter, so the only way a ball wouldn't get launched full distance is if it was directly against wheels before we attempt to launch. Here is a thread with a video of our conveyor in action:
http://www.chiefdelphi.com/forums/sh...654#post465654