It would be unusual for the controller to tell all aircraft to standby (unless he is dealing with other agencies regarding the dump) as we will typically only stop transmitting to the dumping aircraft until they are complete, some of the military assets will even turn RF equipment off so you can't talk to them.
As ANSA said, very common here in the UAE FIR, it would surprise some people to know how much our US friends do throw overboard on a daily basis. Add that to the fact that the level of tanker traffic etc. has hardly abated since the Afghan and Iraq campaigns started. Its no wonder they are starting to experiment with synthetic fuels!
