My aircraft is in the "Mode C without altitude encoder" category. This does not meet the French requirement, but I have never had a problem there. The only time I had aggro from ATC in France was when I was VFR in class G and Seine Information would not accept that I did not have mode C available! The lady controller called me up to apologise ten minutes later and agreed that she was wrong to demand it in the circumstances!
The French rules state something along the lines that it can be over-ridden by a competent authority. I guess this includes ATC and so, whilst there is no guarantee, they will try to accomodate you in controlled airspace regardless.
Before anyone asks, we are waiting for clarification of the Mode C/S situation in UK and France before we decide to spend a few hundred pounds fitting the altitude encoder.