I think the HA booking category is a great idea. We had a HA unit at BLT last month. We talked about asking to switch rooms, but ultimately decided it's vacation, let's go with the flow.
In addition to what Dean said, bear in mind that booking categories would work both for and against members. While people who are confined to a wheelchair are aware of their needs 11 months out, other situations are quite different. I've read many stories over the years along the lines of "my daughter just broke her foot but we still want to go" or "I had unexpected surgery last week and my doctor says it's still OK to travel next month."
When issues arise unexpectedly, excess HA rooms gives the resort an opportunity to meet guest needs on the fly. With separate categories, someone who doesn't need a HA room has undoubtedly conceded to stay in one, leaving a newly impaired guest must either cancel or take their non-accessible room.
Not really sure which system would be better overall...but both approaches have their drawbacks.