That doesn't match my understanding or experience. My understanding is that if a Plat tries to make a reservation (using the online tool or the general reservations agents) the hotel has no direct control over whether a Plat override is available or not. If the Marriott reservations system doesn't have a "waiver" for that night and the Plat in question hasn't already used their Plat override for that night, then the reservations system will offer that Plat a room.

Their only control is to get a waiver from Marriott for the night(s) in question and if they meet the requirements (i.e. the hotel inventory is contracted out to an event(s). ) And even then, the hotel doesn't "turn off" the Plat override, Marriott has to do that as the hotel never had control of it to begin with.

As to what the hotel's general "corporate" rate is, I would suspect that the hotel does set that, but that they can't "change" it just for that "event". I have seen it higher than the specifically negotiated rates I have used and I have seen it lower. So it just depends. But in general, this is one benefit that I think Marriott handles much better than most of their competitors (who have a similar top level elite benefit.)

My biggest issue with this benefit is that when it is being used, the "notice" that you are given is very easy to miss and I don't even realize I just used that benefit to get a room.