Sorry you didn't get an answer yet. I think your question didn't get noticed.
I don't remember ever seeing anyone post that they had gotten a wheelchair when staying at Fort Wildernesss (but, that doesn't mean no one has).
My guess would be they might not because I believe (but I'm not sure) that there are steps to get into the FW cabins that are not wheelchair accessible.
If no one answers you, you could try calling the Special Reservations Line: (407) 939-7807
Good luck and please let us know what you find out.