Thanks everyone! FHR was/is doing 3rd night free at several properties: Sacher, PH, Imperial, Bristol. My plans changed a bit and I'm now travelling by myself, and Bristol was offering 25% off through FHR, bringing the room rate down to $240 a night in a base room, so I switched to the Bristol. I presume I'll be going back with my partner next year, and Sacher/Imperial/PH will be back on the list then. I shall report my experience, and look forward to ChateauMargaux's! Sadly, this means I've also had to cancel at Schloss Fuschl, which I was very much looking forward to, in favor of more modest accommodations.

Data Point: I booked a classic room through FHR and entered my Bonvoy membership number. The reservation is showing up in my Marriott account as a deluxe room. It would appear the property processed the FHR upgrade already - well before check-in. No idea if the Bonvoy gold status would stack another upgrade at check-in - I certainly wouldn't expect it to.