You mentioned that the force required to shift the transmission is quite light. Would it be possible to use the torque produced by a servo motor to shift?
I have drawn a sketch below of the concept. Basically, the inner shaft shifter rod is pushed and pulled back and forth by a pushrod attached to a wheel that is rotated by a servo, in the same way that you would push up/down an aileron on a model plane. The servo can be directly controlled by any PWM output on the robot controller. The servo wheel can be set to any desired rotational angle by varying the PWM output duty cycle. Certain angles would correspond to 1st, 2nd 3rd gear, as well as the neutrals in between each gear.
Do you think a servo would have sufficient power? It would certainly be a smaller, lighter-weight solution compared to using pneumatics, and could probably provide control of 3-4 gears. Because the electric motor in a servo is not particulalry strong, it may be possible to eliminate the springs in the original design. If the transmission is not aligned properly for the shift to occur, the servo motor just stalls, and waits the fraction of a second required for the next gear to line up. It would then rotate just the right amount to engage the ball bearings on the next sequential gear.
