Thanks for the prompt reply, FA. I was aware of the "pop-up" flight plan, but as I understand it is not really a different document, but rather the same information filed in a different way. I have indeed known both Brussels Information and Antwerp tower to file flight plans routinely, after getting all required details over the radio - a heavy charge on channel occupation.
I do remember even international flight plans to be submitted this way.
But a flight-plan filed while flying, through ATC, still needs to be closed after the flight, doesn't it?