[DanDon]

What about using something like a potentiometer and hooking it up to an analog input on one of the joystick ports? Maybe devide the pot's range into a three steps, one step for on in one direction, one for off, and one for on in the other direction?

Or if you're not using all the buttons on your drive joysticks, then use pins 5, 8, 9, and 15 on port 4 which correspong to port 1 trig, top, aux1, and aux2.