I don't know if this is new, but I think it is.
Via b-europe (Thalys/Eurostar reservation page) online it is possible to book reservations for TGVs to/from Brussel for rides to Marne-la-Vallée/Chessy (Disneyland) and Brussel to Aéroport Charles de Gaulle TGV. They ask 20€ in second class and 30€ in 1st class.
Via travel.b-europe.com (for trains within france) it is not possible.
Thank you very much for this information. I have forwarded to the right department, so it will be shown in our website.
Have a lovely day!
Looks new to me. AFAIK these destinations weren't included before.