I had 1D booked, 1F was vacant, the flight was J4 R2 Y5 after check-in closed.

Me: Is business class going to be full?
Concierge: Are you trying to upgrade someone?
Me: No, I'd like to keep the seat beside me empty, if possible.
Concierge: There are four empty seats, and... one person on the list. You're 1D right?
Me: Yep.
Concierge: I'll let them know to keep it empty.

So now I have 1DF.

Certainly not something I expect, but I'm more than willing to ask the question if it looks like J will not be full.