I had this happen a few weeks ago - no seats available for selection (i.e. all seats actually taken - the seat map was presented to me but with none available) in the J cabin on a 767 (24 seats). I was concerned that I might not get on the flight at all as there was no F on that aircraft type.

In the event, at checkin there were 2 reasonable seats available - and apparently no offloads in my favour...