I have thought about using through axle hubs for trailers. I think they come up to about 3/4" (19mm) or so which would be mighty stout.
Another option to consider is a "Lefty" hub used on some of the new MTBs, which is designed specifically for one sided mounting.
As far as the rim, if you're relacing, then you could easily relace onto the wheel chair rim.
Another thing to look at would be cheap fixie rims. They frequently have a deep wall aluminum rim that would give you quite a bit of space to add your driving wheel mount.
Are you using 700c?
Is this a standard chair, or a racing wheelchair?