I built a circuit like this years ago, but instead of a mechanical switch, I had it wired to a pair of magnetic reed relays. One was hidden behind the dash, and it required that a magnet be waved over a specific and non-obvious point on the dash to close the circuit, AND the drivers door had to be closed - there was a magnet in the door and a second reed switch hidden in the door frame. It controlled an electric valve in the fuel line, and if the door opened, the valve closed, and the engine would run out of fuel in around a minute or so (sooner if it was cold and the choke was still partly closed).
I had to turn on the ignition AND close the door before I could activate the switch behind the dash and open the fuel valve. The logic was that if someone caught me by surprise me at a stop light, I could just get out of the vehicle, let the thief drive away with it, getting him away from me, but the vehicle's engine would quit within a few hundred yards - WITHOUT me having to do anything special, like flip a switch, and WITHOUT alerting the hijacker that anything had happened. Just opening the driver's side door was enough to break the circuit, but the vehicle would continue to run on what fuel was still in the carburetor.
Works great - I still have it installed, although I have never yet had to rely on it in such an emergency situation.