Jan, several - pro level - labs do operate in Brussels. See Yellow pages (# 855 in printed version)
The one I know about :

Sadocolor (rue General Tombeur , 1040 ) , E6 / C41 , also for pro-framing : nice people!
Micheli JJ lab (rue de Tervaete , 1040 )
Limelight , 1050

American color (1050 Elsene) : I've heard he's stopping its activity?

I just discovered a small, traditional, C41 , renowed for still using ENLARGER technology!
called "Jazz" rue de Flandre / Flanderstraat

Hope it helps you, Emile (originally from Luik)