I am looking into the possibility of using the "sSpecialRequests" parameter of ReserveTicketsEx(), but I am wondering if there's a way to prevent ReserveTicketsEx() from reserving those seats when no such special request was given.
The best I can think to do to prevent non-wheelchair customers from reserving wheelchair seats is to expose wheelchair seats via a separate mode of sale, and switch to that mode of sale when users click "I require wheelchair seating".