I haven't used this myself, but CO lists SNCF as a travel partner for train tickets booked in conjunction with CO-operated flights to CDG with onward travel by TGV to/from: Angers St. Laud, Avignon TGV, Bordeaux St. Jean, Le Mans, Lille Europe, Lyon Part Dieu, Marseille St. Charles, Montpellier St. Roch, Nantes, Nimes, Poitiers, Rennes, St. Pierre de Corps/Tours and Valence. (As you noted, the air and train portions must be purchased from CO as a single itinerary.)