I hate to agree with BigDave about anything, but he's right on the money here. A frontally-worn fanny pack holster would seem to be the way to go if, for whatever reason, shoulder holsters won't work.
But if on-body isn't an option (I can't imagine why, but you never know), another possibility is that some wheelchairs, particularly powered ones, have a little padded thingamabob in the crotch area to keep the occupant from sliding out of the chair on a sudden stop. Most of those thingamabobs are big enough to conceal smaller mouse-guns if you hollowed them out a bit. Expanding them laterally and vertically could yield enough space for something pretty sizable. The drawback is, of course, that the gun goes with the chair not the occupant, but if you never leave the chair (aside from locked bathroom stalls or your own home) it has some definite possibilities.
There's also the possibility of building a shotgun into the chair itself, though you'd run into short-barrel problems or a lack of concealment depending on how you did it. To say nothing of the problems inherent to being caught with an unlicensed concealed weapon (it's a concealed PISTOL permit, after all).
