This custom programmed arduino leonardo board will enable you to wire 12 pistons to Hauptwerk with 8 outputs on the board for LED output for use with illuminated pistons.

Simply connect the board to USB port on your computer.

Wire piston contacts to 12 input pins on board + common ground.

Wire LED/illuminated pistons to output pins with 220ohm resistors and 5V output.

I can provide wiring diagram and further instructions if needed.

See pictures of my own hauptwerk organ which uses one of these custom programmed arduino boards to encode button pushes and decode hauptwerk messages to light up illuminated pistons.