Jan,
The French have stated that they will seek to increase the microlight MTOW to 500kg, as that will allow them to preserve their 'declarative system'.
I suspect they would not go for the increase if they did not have some sort of assurance from the DGAC that it would be accepted without affecting the current regulations.
Not all foreign microlights require overflight permits in Belgium, homebuilts are exempt.
I don't think this opt-out is a problem for those who want to preserve the 450kg limit, as Annex I (ex-Annex II) remains unchanged.