I recently flew into Le-Touquet whilst their ATC was closed.   I was reminded by the FIS just prior to landing to make sure I closed my flight plan and attempted this using the general French phone number immediately after landing.  I got no reply from the number.  

It already does. When you land, after an international flight, you get a big banner on screen saying something like "Welcome to France. Don't forget to close your flightplan."

Thanks Tim, if a little embarrassing I didn't notice it.  

Looking back at the cockpit GoPro footage of the landing above, I don't seem to have been presented with any sort of message as you suggest Tim.  Are you suggesting something like a Controlled Airspace or Danger Area warning that sits in the middle of the screen?

The message is triggered when logging stops. If SkyDemon has started your log automatically on takeoff, as it does for many people, it'll therefore appear on landing. If you've started logging manually then it should appear when you then stop logging manually. If you just go straight for Stop Navigating you'd probably never see it, because such banners only appear in navigation mode.