I think a wheelchair bound actor as King Arthur in SPAMALOT would be HILARIOUS!
In all honesty, I could see it working in about 50% of the plays I thought of at the top of my head. It's an interesting thought. It DOES change the character. I guess what matters if it that change actually changes the show itself, and if it does, would it negate the point of the show?
"The Spectacle has, indeed, an emotional attraction of its own, but, of all the parts, it is the least artistic, and connected least with the art of poetry. For the power of Tragedy, we may be sure, is felt even apart from representation and actors. Besides, the production of spectacular effects depends more on the art of the stage machinist than on that of the poet."
--Aristotle