In the Netherlands we have a "Recreational PPL", which is more or less comparable to the NPPL in the sense that it is only valid within the Netherlands and requires less experience hours than a PPL. When we go on a club trip to a foreign country, we typically ask the country where we're going to for a few waivers. One of those is for RPL holders, assuming they meet all the Dutch requirements with regards to restrictions, medical, currency etc., to fly in that foreign country. These are typically granted without too much problems. You might want to try the same thing in the US.
I've been trying to find a way of contacting the French SIA for exactly this reason. Do you have an address?
thanks
Bernie