Given that you presumably don't want to sit in the lobby for 12 hours, it would make sense to call the property directly and make the appropriate arrangements. Perhaps they can acommodate the request, perhaps they will charge a full night, a partial night and perhaps they simply don't have room.

I'd certainly want to know ahead. FT can't help here because ever other property in the world could acommodate an FT member, but your property happens to be sold out the night before.

If it's a place you frequent, don't forget that this is where very good tipping gets you results.