The idea that this is unethical is silly. Accessible rooms are rarely, rarely used for their intended purposes. Like others have already mentioned, a note on the reservation is good enough to let the property know that you're good with another room type if it becomes available the same as a note on the reservation is useful to let the property know that the accessible room is something that you need. FWIW, it's fairly common for hotels to make accessible rooms available to 3rd party sites to book on the cheap. If I'm travelling on a team member rate regardless of brand, I expect to get an accessible room regardless of the occupancy rate of where I'm staying. I've been getting accessible rooms give or take 50-60% of the time now for years.