Sorry, late response, but most countries (including France) require a work visa for things like unpaid internships, or being compensated in any way. Paying for board/food counts as compensation.

Look at it from the perspective of the US -- you can't just recruit programmers from India, have them come over "on vacation", pay them in room/board, and expect to not have any problems with border control. Work is work.

I don't know the official answer, but given that it's all under the table, short term and no money is being exchanged, I think you'd be crazy to even try getting a work visa. I'll be honest that it didn't even cross my mind.