One of the things for consideration would be the tiedown/lockdown necessary. I can visualize having a Doom Buggy that was equipped to handle a "standard" wheelchair (whatever that is).
The chair would have to roll in with the wheels going inside of guide tracks, the CM throwing a lever, and clamps locking all four wheels and also clamping to the frame. Then turn the base 90 degrees so the wheelchair faces forward and it locks in place.
Getting off the CM would have to rotate the base 90 degrees, throw the lever the other way to unclamp everything, and the wheelchair would then be backed out.
When I say a "standard" wheelchair, I would mean an adult chair of the variety that Disney rents out. As the latching mechanism would be based on the dimensions of that chair, it means that if someone rented a wide chair from Disney that would not work. Clearly it would not work for a transport chair, a power chair, or a pediatric chair. And if Disney ever changed the brand or model of the wheelchairs they rent out it might mean that this would not work.
I could see the possibility, if such a Doom Buggy were built, to require that people must transfer to a special wheelchair from whatever chair they were in to be able to go on HM.
(Note, this is just conjecture and is not to be considered anything official or anything that Disney may even be thinking of.)