Yes, as I mentioned in my earlier post, France registered a difference when they originally signed up. They were the only state to do so. Other EU member states would have to register differences to abrogate their original ratification and then stop issuing at least some licences.
This is, as I also mentioned earlier, contrary to the spirit of ICAO Annexe One's intention that certifiers demonstrate competence under state supervision. France does indeed seem to have difficulty with leaving its old system behind.
As to legal measures, anything can be challenged at state level. Its what sovereignty is all about. EU law normally takes precedence and states are obliged to incorporate EU law into their own legal systems. However, EU states retain the right not to adopt mere agency rulings, unless they are ratified by the European Parliament.
This issue concerns an EU agency ruling that would overturn a clause of an international treaty already ratified by sovereign states and it is a moot question if EASA - or even the EU itself - has authority to do such a thing. The EU is not a sovereign state and has no power to sign or ratify an international treaty.