The following Belgian petrol stations and air de Services allow a hitchhiker to cross over to the other side. The list is far from complete, and currently only contains entries collected by Prino.
E40
Groot-Bijgaarden, just before the Brussel ring when coming from the coast
Heverlee, after exit 22 and more important after the exit towards Leuven
aire de Tignée, between Liège and the German border
E411
aire de Bierges, near Wavre between exits 5a and 6