Treeless often does not leave a channel over the spine to protect the bony column from being weight bearing. No tree harness can work ok on 4-wheeled vehicles with lightweight wooden shafts, such as older buggies. The Amish use it that way and call it “buggy harness,” which is your usual clue it is treeless. Modern 4-wheelers with independent shafts, can put a good amount of weight on the harness saddle, so they need a harness with a tree in it to man age that shaft weight. Most inexpensive, box pony harness is treeless. Saves costs and lots of people don’t know it is for 4-wheelers.
I would recommend a harness with a tree for a pony and 2-wheeled cart. There is ALWAYS weight on the shafts of a 2-wheeler, with pony being the supporting point. A wider saddle will spread that weight over a larger area of the muscled back than a narrow saddle will.
Width also helps protect the back better as passengers enter or leave the cart, adjust their seating during travel. Perhaps you could have pony owner hold level shafts where the shaft loops will sit on harnessed pony. Warn them to hold TIGHT as you get into the seat, lean forward and back to demonstrate how shaft weight changes on the pony with ANY movement by the passengers. Do be ready to jump aside in case they can’t hold shafts in place! They will always be surprised at the shaft weight, may let go!!
That is what pony will be feeling, so his back, spine, need the saddle with a tree to protect him. Spinal column was never designed to be weight bearing in work.