To avoid this problem, which can be downright dangerous if you should, for instance, suddenly lose pressure on a zero-turn riding mower on a hillside (happened yesterday), a fundamental change in how the throttle is operated is in order.

Make the throttle control a dial, preferably with positive detents. User-adjustable detents would be ideal for those of us who want to keep all control over this sort of thing. To prevent a branch from accidentally tripping the large knurled knob, have a guard around the knob itself, just a ring of heavy rod that leaves plenty of clearance for gloved fingers. The average mower operator doesn't alter his throttle setting after having got the engine going, so this shouldn't be an issue in operating the mower.