http://www.conferenc...om/brussels.htm has a list of upcoming conferences in Brussels. Perhaps we can steal their venue ideals.
Didier Coeurnelle suggests: http://www.ihecs.be/
This has a large lecture-style room that holds up to 400, at €400 for "an evening", though Sven thinks that other time periods (ie, Saturday and Sunday daytime) would be negotiable. The main downsides of this venue would be the paucity of catering facilities and the inability to convert the lecture room into something suitable for roundtable style discussions.