Agreeing with the others.
When you work with Special Reservations, they actually block an accessible room for you, so it would not be a case of you got there too late. A specific room is assigned to you when you make the reservation for the accessible room (that might be the easiest way to think about it - you are not making a room reservation; you are making a reservation for an accessible room.
I'd call Special Reservations yourself and ask them to fax you something (if you have a fax).
We have not stayed in the regular WDW resorts, but our confirmation for OKW (with Disney Vacation Club) has said "Accessible Room Guaranteed"