I don't know if I posted on this in the search you did, but on a trip in Maine, I saw a group with 3 people in wheelchairs. They had 2 counselors for each disabled person. The kids had non-motorized wheelchairs. On the water, the chair was lashed into the center of the canoe and the kid was loosely strapped in wearing a PFD with leg straps they couldn't come out of. The strap was loose enough that in a capsize, they would bob up and out.
On portages they used long wooded poles about 10ft long, 2" in diameter, which the lashed to arms of wheelchair on the portages. They would roll/lift as needed. Granted they weren't mountain goat ones, but not flat paths either.
We saw them a couple of different times one day, and they had it down to an art. They even had rods rigged to the kids wheelchair so they could troll as they moved. I was quite impressed. The canoes were Old Town Royalex trippers, heavy, but very stable.
I know your not supposed to have any type of cart in the parks, but IF you could get permission, I think one of those collapsible aluminum game carts like they haul deer with would be perfect for hauling folding wheelchairs/campers across a portage. One pulling cart, another assisting from the rear. They fold flat, have puncture proof tires and carry the weight balanced.
That way you wouldn't need real strong porters to carry like I first mentioned