The fundamental problem here is that hotels don't currently have any real incentive to seek volunteers. There is no hotel equivalent of a VDB because there is no published measure of IDB. The airlines have to publish bump data so it's (usually) in their interest to find VDB's instead of IDB's.

Technologically, hotels could seek VDB in this specific case. That would be the most fair thing for everyone involved.

I also find it unfair that Starwood doesn't honor their walk policy if the hotel notifies you early. The net result is the same: you made a guaranteed reservation and the hotel didn't keep its end of the bargain.