I can only tell you this:
I was going Praha - Barcelona in July and due to some construction works in Germany I missed my booked TGV so I had to go via Basel and Geneve to Lyon to catch another TGV there. I had no reservation so I wanted to explain my situation to the conductor but she only told me to get on the train (to the bar carriage) and wait there.
Quite some time later (after Valence) she finally came to me and I got a "bulletin" which cost me 15€. Since I had no change and also nobody else of the train crew did I got away paying only 12€...
So, I think you can just hop on the train in Bruxelles and pay the "on-board tariff" - if it is worth it forth you. The train I went on from Lyon to Perpignan was quite full, I cant say if it was fully booked but there were maybe 3, 4 others who also didnt have a reservation.
In the end I eventually got a seat for half part of the journey - but be prepared that it might happen that you have to stand the whole time in the bar carriage if you hop onto a booked train.