Quick report for anyone else in the same situation reading this.

I turned up to check-in at the AF desk, and was offered middle seat 8E. I had a bit of a grumble about this as it wasn't really my fault given there was no way to check in online, and eventually they upgraded me to premium economy in 3C. This was obviously down to a) the platinum card, b) having a friend flying in premium in 3A and c) luck.

Lounge access, as suggested by SheBangsTheDrums, was fine. So all in all not a bad experience but I'll try to book direct with AF in the future to avoid check-in / seating issues.