I did some reading on the subject and it appears it's not easy to walk in a French telecom and buy a prepaid sim like you suggested. Some suggestions that people had was to open a bank account in France (seems like even that's not so easy as banks seem to want income verification), or if you have a friend in France or know anyone there, you could ask them to buy a Micro Sim for you. Do you know anyone in France?
I've read the terms of service for one of the 3 providers and indeed you need a French credit card in order to buy connection time for your iPad. So you'll need to have a "local" friend buy it on your behalf.
As you mentionned you'll be away from major population areas, I suggested before picking one of the tree providers to first check the 3G coverage.
Bouygues Telecom network:
SFR network :
Orange network :
As for MaxRoam, I'm not sure they support data in France. You'll have to check this first and also the 3G network coverage.
MaxRoam SIM for iPad didn't work. I'd recommend staying away from MaxRoam for the iPad SIM. Product would indicate available networks (I was traveling in France), but trying to use Maps or Safari resulted in "no data service available" error messages. Trying to get support in a timely fashion from MaxRoam while traveling was a fail. Additionally, the iPad SIM is shipped with a page of instructions that are for a cell phone and completely useless for the iPad. Finally, since returning from my travels without being able to use the SIM, I've received multiple, different suggestions for configuration settings that were never provided or available on a support site that would have helped BEFORE traveling. One of their after-the-fact suggestions is to ensure your network settings are correct by following a URL - which doesn't really help when you're using a SIM that doesn't provide a Web connection in the first place....