Hey RK,
A few addionnal comments...
Marseille used to be my hometown. I ve never kited there but, based on the geography and the main wind direction in this area, I would select:
1- Hyeres. There is a north-south peninsula. Sand beach. Nice surroundings. In this area as well: La Ciotat, Six Four, St Cyr...
2- Marseille, at 'La pointe Rouge': famous windsurfing spot when I used to live there (now Kitesurfing spot I guess) in Marseille. I heard this is a spot for experimented kiters only (limestone coatline, boats, people, everything you may want to avoid)...
3- Corsica is a very very beautiful island, definitely worth visiting. Just avoid the bombings!?!
It s very likely to be sunny in this area in September. And whats best is that the wind usually blows the clouds away, so 'windy' means 'sunny'! This area is known to be a windy region in France.
You could also try west of Marseille (Port St Louis, Port Camargue, Leucanau). I am not familiar with these locations...
I also lived in Brittany for a few years. There is a very beautiful rugged coastline. But September is the beginning of the stormy season. It is very likely to be cold and rainy

. Be aware of very strong current and tides (6 m in Brest). Brittany being a big penninsula, you could reach the right deserted spot for any wind direction within 45 min of driving from Brest.
For live information about wind in France, try:
http://www.baston.fr/internet/asp/cartr.aspCheers,
Fred