SATCO
The usage restrictions are a little more specific than I implied in my first post.
Dunkerque is not a publicly licensed aerodrome. Unlike the UK, the business of licensed or unlicensed refers to who may legally land there. With a very few exceptions, landing an aircraft in France other than at a publically licensed aerodrome is an offence in its own right.
Many smaller aerodromes in France are "licensed for restricted use", which allows them to avoid meeting the full ATC/FISO/FSS and other requirements that the French impose. The downside is the use of such aerodromes tends to be restricted to based aircraft and those operating from similar fields with a reciprocal arrangement in the same region. Dunkerque is just such an example.
Presumably, the aerodrome administrators have obtained a waiver from the regional DGAC for the event that you have been invited to. It would definitely be worth your while checking though and getting some written confirmation before flying.
It is a great little aerodrome, and well worth the visit if you do get the chance.
2D