I had a Zippyy ball handle joystick knocking around so decided to use another Raspberry Pi as a controller/relay. Each axis of the joystick closes a simple mechanical switch when the handle is moved in that direction, and this is detected using the GPIO pins.
All communications between the two Pis (the A+ in the Diddyborg and the B+ controller) are via wifi.
I used a Slice of Pi board for the connections between the GPIO pins on the controlling Pi and the Joystick. I also had plenty of space to add a bi-colour LED which I use to indicate when motion commands are actively being transmitted to the diddyborg.
The code that runs on the controlling Pi and the DiddyBorg Pi is available on GitHub.