One problem you're up against is that France pretty much shuts down over the school summer holidays - certainly most cultural groups do, and even real charity work (soup kitchens and the like) go on minimum service for the months of July and August.

Volunteer work is not as popular in France as in the "anglo-saxon" world (well, that's what they call us here), and actually volunteers are strictly divided in the regulations into the "volontariat" which includes things like volunteer firefighters, who have their own statut and rights that go with their service, and the "benevolat," who are just those who do stuff for free for local associations with no particular acquired rights connected with their "work."
Cheers,
Bev