@acidbass I've asked two friends living and playing in Brussels, but for now, the only answer I have is this society, which seems to be the only one remaining in Brussels city...
Poor offer I know, but Brussels has never been a good place for musicians to hire backline.
So here's the link: https://locinstrum.be/
The son of one these friends is into events management (and a musician too) so I'm waiting for his answer. 🤞