Yes, I've seen this happen with travel agents in Hong Kong a few times. A couple of years back (when business was good for CX), the flight to Rome was zeroed out in all fare sub-classes for our preferred departure date during the summer peak season. Not sure, but I seem to recall the flight was so full the waiting list had also been closed. Anyway the agent--with whom we did a lot of business--got her GM to take up our case with Cathay, and they forced open two seats for us in one of the economy sub-classes. Not the cheapest (it was K I think)--but we were very happy to get any kind of seats. We were MPC Green at the time, so there was no issue of any kind of seat guarantee.

I've also seen this happen with other airlines, such as Emirates, where a good agent with the right connections can get you instantly confirmed even on flights with a long waiting list.