No idea about clubs etc, but some interesting (high end) street action at the following time and place : on Av louise, from 10 pm till 4-5 am, between rue lesbroussart and rue du beau site (check places on maporama.com). I drive there regularly and always see 2-5 girls, 20-22 yo, romanian I think, speak some English & very little French. Pros but 7-8/10 in looks. Charge 100+. Never tried (don't like to hunt on my home turf) but probably less of a rip-off than most clubs etc.
Rue d'Aerschot along Gare du Nord (this is the right spelling) you'll find a sting of bars, a bit like in Amsterdam (but not that many of course) with many east-europeans etc. Beware of arabs & albanian youth in the area, can be dangerous at times if you arent' street-wise.
Usually a few SW on Bvd Albert II, both lanes. There used to be lots of them, also in neighbouring rue du commerce (veru close to Place Rogier, hence the confusion of a post above on this thread), with many cars stopping by and picking them up, but haven't checked in a while. I think the police have been pretty active in harrassing them, as most were illegals brought over by pimps who bring them from place to place. Still plenty of possibilities though, esp if you have a car. Rates 50+.