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Agree that with the coach company in advance. Give the flight number, so the driver can follow the arrival.
According to Brussels Airport, the bus station, the taxi zone and the place for coaches are in one zone in front of the terminal, together with the free drop-off for departing passengers; the coach car park is nearby. That is in a press release from the airport from 2018, and things may have changed since. Have the coach company check the place, and give it your flight number.
Not in Zaventem itself. If the coach goes on into Brussels, it does: the whole Brussels region is a zone, and a diesel coach must be at least Euro 6 there.