Ya thats what I want to know too,
Looking at Ultimarc website under the opti-pac diy optical sensor page there is a diagram for a happ optical pcb.
Don't know if these are the same for the optical joystick, but most proberly are.
Talking about rotary joystick has anybody got it connected up to the i-pac with another joystick (non happ), so you can use any of them.