Posting real world example here as the most recent thread on the subject. Received an OLCI offer on SEA-CDG from PE to J for $535 on a flyingblue award booking. Business redemptions were very mileage prohibitive on my travel date (never less than 150k), so definitely worked out well in this case.

For those wondering how this works, I checked in on my phone via the AF app (30 hours before not 24 hours!), and then it showed I had an available seat selection for $535 and sure enough I could select business seats and check out. Only a few seats remained so it doesn't seem like they hold any back.