In addition to allowing a joystick button run a script, why not let it trigger a custom event of any name. If the joystick moves left, the user could say this action triggers "port-side" or whatever they want to call that.

You could then setup on or more listeners (on say an h-bridge movement control AND servo moving robot head) to listen for "port-side" and respond accordingly... The response of course would vary with the item listening.

Pressing the left arrow on the keyboard could be assigned to the same (or different) event as moving the joystick left.

I think this would be a very powerful additional feature.