Modifying anything would take a tall vehicle like the vans mentioned.
Then think in terms of the stand-up amusement park rides when thinking about the seat size, shape, and construction.
It sounds doable.
Check with mobility mod shops like those who fabricate damn near anything for adaptation. From wheel chaired drivers to hand controls and lifts, these people have the fabrication expertise.
This sounds like a job for Monster Garage and other simmilar TV shows.
e-mail them with this challenge, maybe you'll get on TV.