If it was like the lanyard on outboard motors, it probably would not be enforced like that rule is. A PWC or outboard will still run with the lanyard not attached to a person, but enforcement personnel look for that violation, just like seat belts on cars. It would be a little hard to enforce on a tractor.
I guess you could run a jumper wire through the seat interlock, and attach that jumper to a short lanyard attached to a belt loop. If you actually used the lanyard and then fell off, it would pull the jumper out of the seat switch and stop the engine.
