Correct, I forgot about Belgium, which as of AIRAC 04-2015 from 2015-04-30 say in AIP ENR1.10:
A flight plan shall be submitted prior to operating:
a. any flight or portion thereof to be provided with ATC service;
b. any flight above FL 660;
c. any flight at night, if leaving the vicinity of an aerodrome;
d. any flight across international borders. Flights remaining within the Schengen Area do not need a flight plan as far as the Belgian part of the Brussels FIR is concerned (for requirements applicable in other Schengen States, please consult the relevant AIPs).
So, if you take the C150 above FL 660 you still need a flight plan ;-).
(Interestingly no upper limit, so do NCC1701D and even NCC74656 need to file flight plans when over Belgium?)