Steve, there's an article by Nick Herrin, on page 38 of the Winter issue of TPS all about it. Goes over the legalities and the politics of it. Also a companion article by Chris Fellows about the French Ecole Nationale de Ski et d'Aplinisme (ENSA) which is the national school. Nick and Chris visited them while at the PSIA National Academy last spring in Chamonix, also a couple of Brits who own a ski school there who shed light on all the legalismo involved.
Short version: Pretty much a closed shop over there.
Looks like you need all of:
1. Level III as the only ISIA-accepted PSIA cert.
2. Right to work in the EU (ain't gonna happen for Americans unless you have Irish ancestry and thus can get yourself an Irish Passport under a right-of-return deal)
3. Euro Ski Pro certification through ENSA or through the national schools of Germany, Austria, Belgium, Denmark, UK, Spain, Italy, or France.
4. Registering with the French Government Ministry of Sports.
And probably a lot more stuff.
The Brits, even though fully legal under all those rules, get pulled over by the ski cops all the time for "your papers please".