In my past life as a non-rev, I saw at NRT the boarding come to an end for NRT-SEA, and then when the last non-status Y passengers were about to board, the gate agent simply asked if anyone would be willing to give up their seat in exchange for C on a NRT-SFO-SEA routing.

Mind you, there had been people who volunteered for VDB, but the agent just skipped that process and gave it to the last people to board.

So I think that if a plane is late for departure, they might skip the formalities. I think the gate agent was able to convince these pax to give up their seats on an oversold NRT-SEA without issuing any compensation like travel credits. Just a C seat on a one-stop routing is all it took!