I do not know of a tractor with that kind of access. I know there have been discussion here before, about adapting a tractor, but the person had more mobility. That was 5 or more years ago though. You might use the search on this site.
My NH TC40 is HST, and hydraulic steering. It would not be hard to set that up on a joystick; all the skidsteer companies already do that. It would cost $$$, but could be done.
I can not see it done without a lift to transfer from the wheelchair to the tractor though, like the one my Grandmother uses to get from wheelchair to pool at aqua-therapy. The one at the pool is more of a small crane/boom; we have to put a net in her chair, and chain that to the boom.
A skidsteer(or compact track loader) would be great, because it is already set up with controls. But, most would be impossible to get in/out of, even with a lift.
What you want is certianly possible, but I fear it would be prohibitively expensive.
