As a purely engineering comment, provided you anchor the bench to keep it from moving relative to the tractor, I think your solution can function as intended.
I've seen people use unguarded piledrivers on 3 point linkages to drive fence posts, Jim - they do a beautiful job in no time at all, and it's all good unless you get your hand in between the driver and the post. There are lots of pieces of gear used traditionally on farms that would make an OH&S inspector faint or run screaming from the scene.
I'm not criticising your judgement, but this is a public access site and we need to take a responsible attitude.
