Not that I am proud of it, but y'all had better be warned: I received mail today, via my aeroclub, to the effect that, for flights between Belgium and any non-Schengen country,
1) only the aerodromes EBAW / EBBR / EBCI / EBKT / EBLG / EBOS can be the point of entry/exit
2) filing a "General Declaration" document is mandatory - as I read it, for arrival and again for departure.
Fazit: better stop in France if crossing the Channel. And if one still decides otherwise, here's a convenient place for filing the GenDec document:
www.generaldeclaration.be
Most of this must be in the AIP too, somewhere in GEN1.2