What do you mean by "unable to book Berlin round trip”? There's no quota on ICE reservations and reservations on the ICE to Berlin are optional anyway.
It is possible to do a round trip from Brussels to Montpellier and then to Berlin. The TGV Brussels to France has a fare for passes only valid in France (the "Pass 2” fare). Currently, the price is the same as for passes valid in both Belgium and France. That means that you can do the round trip to France without using your 2 inbound/outbound days. Your inbound/outbound days can then be used for the round trip to Berlin.
You can book the "Pass 2” fare by phone via SNCF or NMBS or at the NMBS international ticket office. Neither have a booking fee.