We do a similar thing with BMW MSD ecu's with the steering wheel buttons. Can switch from stock and stage 1/2/3 with the volume button on the wheel and rev counter shows which map you are on, 3k rpm = stage 3 0rpm = stock etc... Double press the volume down to go back to normal operation.
Maybe an external switch wired into brake light switch circuit could work? Plenty of blank switch positions available to utilise on the mk5 with an OE switch.