Using a robot chassis to control a remote robot chassis requires position sensors in the controlling chassis. Because the robot frame was designed to accommodate servos, the easiest way to add position sensors is to buy a bunch of cheap servos, and modify them to provide position feedback. It turns out you can buy such things from Adafruit (see https://www.adafruit.com/products/1404) but at $15 a piece, these seems a little pricey. All I need is a servo sized box which turns a potentiometer to produce a voltage proportional to its position.
Instead I bought a bunch of cheap servos (such as these http://www.ebay.com/itm/10Pcs-High-Speed-Torque-Standard-Servo-for-S3003RC-Car-Helicopter-Airplane-Boat-/301763979131) which cost me $3.50 each and modified them to provide the position feedback.
Modify these servos is really simple. First unscrew the based the servo unit by removing the four screws.
This exposes the control board and motor.
If you lift the control board you'll find a potentiometer underneath it. This is what we want access to. There are three wires connecting the PCB to the potentiometer. We need to sense the voltage found on the center wire.
As with all servos, these are operated using three wires: power, ground and control. I still need power and ground for the servo (we need a voltage to be divided by the pot) but I no longer need the control wire - so I reused this to provide the position output instead. To do this I unsoldered the control wire from its current position on the pcbs, and re-soldered it to the central lead from the potentiometer.
Below you see the three wires - ground, power and control (white):
Here the white wires has been moved to a pin which is connected to the center of the potentiometer. Fortunately, in this servo at least, the potentiometer is attached to an easy to reach point on the PCB.
Now, when the servo is powered, the control lead will provide a voltage proportional to the position of the servo.
Finally, because we're only using this servo for feedback, I disconnected the motor so it would never be powered. You can do this by simply cutting the wires, or you can unsolder them (which is what I did).
All that remains is to pop the top back on, and we have a position sensing "servo" which can be used in the robot arm controller.