Well. here are my observations... The use of a seatbelt without a rops structure is generaly a no-no.. with the seatbelt on.. the passanger has no way to dive off the tractor in case of a flipover.
Also.. there should have been footpegs or a footboard, instead of relying on feet resting on the rear fenders of the tractor.
The attatchment points on most lawnmowers will be limited to a small tounge, and/or perhaps some small area made ofr a bracket for a sun shade. That IMHO will be the limiting factor for carried weight.
At least with the mower pictured.. it looks like there is little chance for a backflip on level ground.. that mower has good weight forward, and low profile. However a mound or hill may cause problems on that mower.. or another mower with different geometry may also have problems from the getgo.
I made a similar kind of buddy seat for my 66 IH cub.. it was a plain pan seat, braced to the fixed drawbar, and to the rear of the tractor.. it has foot pegs, and a fixed 'joystick' for the passanger to hold on to. Geometry of the tractor is more than sufficient for ballance, as there is a belly mower and front blade on that tractor. I copied the design from one i saw on a massey harris pony, which looks very similar to a IH cub. It was at a FL flywheelers tractor show and was used as transportation. Many of the shows allow golf carts, lawnmowers, and tractors under a certain size to be used as intra-show transportation, with the rule that you must have a seat for each passanger.. no pasangers standing or 'clinging' to the tractor.
Soundguy