At the entrance a large sign instructs you to enter your mobile number to receive a passcode to access wifi.
However as I was in flight mode to avoid roaming charges as Switzerland is not covered by the EU zero roaming charge agreement, no message came through (it did so when I left flight mode on arrival in BRU).
Only on leaving the lounge did I notice a column near the washroom entrance where I could scan my boarding pass to receive a printed passcode.
The sign makes no mention of this possibility.