Never mind, we thought - although 'plusiers aeroports regionales sont fermées' (pardon my French), perhaps we'd be lucky. And if we weren't - well, we probably have to turn up to check-in in order to qualify for refunds, alternative travel, whatever.
We got out to the terminal a little on the early side. Check-in itself was quite smooth, and didn't mention any problems, so we wandered off towards our gate.
I have never seen a proper airport so quiet. There were no visible aircraft movements. For the next hour, we didn't see a single aircraft land or take off.
Eventually, we did hear a boarding announcement for an Air France flight, and our lounge did start filling up. Checking the departure boards, it seems that most traffic there is morning or evening, and only about 20% of flights (French internal ones) were actually cancelled. And so, although our inbound flight was a little late, we got back to T5 without fuss.