Thanks very much to both posters for this information. I now recall the use of the Piedemont Lounge.
But hang on -- on the TRN lounge page on your link, when I insert Gold Card holder and Economy in the form underneath the lounge description, it says
Nous vous accueillons dans les 530 salons Air France et SkyTeam lorsque vous voyagez sur un vol international AIR FRANCE, KLM ou SkyTeam. (my bolding)
Now, either this is a recent change or it is wrong (or I have been dreaming). Surely Gold card holders are admitted into lounges, with guests even, when on a domestic flight in economy?
Also, thanks Goldarak for the helpful information regarding ORY. I wasn't aware that MPL was la Navette and thus Hall 2. I see a fair number of aircraft with those beautiful orange letters HOP on them at MPL. But then, my latest flight MPLCDG was operated by HOP, so who knows ...