Ok, de-energizing the fuel shutoff valve works fine for me (2001 TDI). That's my plan.
I turned off the "ignition", and clipped the lead to the solenoid (don't ground anything out!!) so I could see what happens when I disconnect it cleanly while the car is running.
The engine shuts right down, as expected.
The solenoid is a somewhat hefty circuit though so I will rig up a microswitch on the shifter and control a NC relay with it.