score:4
I pass through the Brussels-Midi station quite regularly. I cannot say for sure, but it is a large station, including the Eurostar terminal, and so I imagine that it is possible to get hold of cart if you ask someone when you get to the station (if you can speak French this would help).
I would advise checking exactly how much time you have at Brussels-Midi. If you have 45 minutes or more then that should be plenty of time to find a member of station staff/a cart to help get your luggage to the relevant platform. If you have less time then it could be tricky. You could probably ask the ticket collector on the train to Brussels and they could advise you what to do when you arrive.
The Man in Seat 61 may have some helpful information on his luggage page. I would be surprised if Air France will forward your luggage for you, but I guess you should ask them to check.