I don't know if the ign switch would simply kill power to the ECU? Seems a bit brutal!
Although the hdi may seem complicated it's mechanically very simple and akin to petrol injection. Albeit with very high pressures and direct injection from 4 injectors. The link above gives about the best information I've found, with some useful schematics. (If you navigate back to the parent pages there's also some VW TDI stuff).
If the ECU "thinks" rail pressure is below 120 bar then it won't allow injection to start. Try grounding that signal and see if it also forces a stop? Might result in a fault code (but in my experience the light goes out after 200 miles!)
