I see. Sounds like i may be stuck putting a potentiometer on the shaft. If thats how it worksI looked for something similar many years ago. There maybe digital servos that can do this. Unfortunately the open servo project died or that could have been modified to do what you want.
Well. What i would do is just use a potentiometer that i can add to the shaft of the servo. I had actually just started looking for one.Normal (not digital) servos have no way of sending information back to the controller. Can you have two identical rigs, one with servos and one with servos without motors. Move one and the other should follow. The empty servos will need matching to the coresponding servos but it should be doable.