A sub-IR IMC privilege would work well in France.
Put a limit of FL110 on it, that keeps it below the major Class D base but would allow transit of the Class D airport zones.
Presently, VFR traffic is completely routinely allowed to pass through all this airspace which is for the most part devoid of traffic.
There are just a few mountainous areas where FL110 would not be enough, but not many.
And FL110 is a reasonable level for which (or below which) one could file meaningful IFR flight plans.
Mode S would be mandatory of course, together with BRNAV above FL095.