ING,Fortis,Dexia,KBC are the biggest banks in belgium.they may have quant positions.But if you don't speak french and dutch you might have a lot of difficulties getting a job in belgium.you should at least speak french to get a job in belgium.except those you may try Petercam.It's a hedge fund.
www.petercam.be. quite nice place to work if you can get in.