That 1 ball storage in the back is pretty clever!
Take a look at this robot back from NbN season, which stored the balls in the magazine:
6210z Post State Robot Reveal (although it was later replaced with different design for Worlds)
I think, there might be a relatively easy way to store two balls in the back of the robot (and third pushed out when overflows) with the ability to retrieve them back into the main loop when necessary. You could ratchet the top roller motor to select whether the bottom roller takes the next ball from the intake or backpack storage. The tricky part might be the latch mechanism that allows top roller motor to momentarily activate it and the switch back to forward direction.