What would be the problem with say (coming from the UK) turning up just before KONAN, FL074, and ask the Belgian ATC for a transit via X Y Z?
I see it is Class C but they never seemed bothered when I used to do that.
My feeling is that ATC are more bothered by low flying traffic asking for transits. If an airliner has to go around from an instrument approach, he will climb to maybe the ILS platform, say 2000-3000ft, and then have another go. So low level traffic is a problem. But if you fly high, ATC has easy options for separation.
just call up Brussels Info and ask for a transit of the general class C at some suitable FL above 4500 feet. Below that it looks absolutely hellish
Exactly...
Obviously you can't do this if it is OVC015 and 10000ft of solid IMC above that 
What would be the problem with say (coming from the UK) turning up just before KONAN, FL074, and ask the Belgian ATC for a transit via X Y Z?
That's exactly what I did (coming from Germany at FL65 requested SPI CIV KOK, was given dct KOK FL60, with one small deviation on vectors for paradropping near [Namur?]). There was very little traffic at my level (nobody crossing VFR on frequency, although plenty of CAT climbing and descending).
Despite passing right between Brussels and Charleroi, and having their arrivals pass below me by 1000ft quite regularly, ATC didn't seem bothered that I was there.
Be prepared to get a "negative, transit not possible due to [bullshit excuse]"
Sounds like the SOP in Australia!
