> get you jammed in gear
Terrifying prospect. What are your options in that situation? Would turning off the ignition have any effect?
Press the clutch, brake to a stop.
Unscrew the bolts holding the top of the gearbox where the selector mechanism goes, lever the main gear back into place with a long screwdriver, and then use the screwdriver to tap the mainshaft nut around until it's reasonably tight again. Then drive it home where you can fix it properly.
At least, that's what I did.
Just stomp the clutch and shut it off