I made an enquiry a few months back to the DGAC in Paris. I also have a JAA CPL with all the trimmings and a job offer in the French West Indies. As I simply couldn't afford to renew my IR/MEP I asked for a conversion of my JAA licence to a French CPL. The UK CAA had given the chief pilot/company examiner written permission to carry out my IR/MEP renewal in the islands but the French still felt that this wasn't good enough. Don't really know why one has to convert a JAA licence to a national licence in a JAA member state but would love to find out. No decent answers from the relevant authorities forthcoming as yet!
What this all boils down to is that even though all the JAA member states have signed up to the same thing and knew what they were letting themselves in for there are still "differences" from country to country. The point of JAA is that the bearer of such a licence should be able to use it/renew it in any member state, to the same standard and that all member states will recognize the validity of the licence. Working rights/language requirements are another matter (and very popular as a means to keep out those they don't want, which for France seems to be almost everybody).
I fear that it will be some time before the rules are applied to our benefit instead of the reverse. Perhaps many of the signatory states rushed into it for political reasons without being adequately prepared to implement.