Upvote:6
I don't know what your question is, if you want to commute on Lille-Bruxelles or if you want to commute on the Eurostar.
For Eurostar, most likely I would say this is not the case. On the SNCF page for commuters, while there is a link to actual commuter pages for other trains like TGVs and Thalys, there is no such thing for Eurostar, just a link to the frequent traveller program.
As for Thalys, they have frequent traveler passes (called ThePass), with discounts up to 50% offon all your trips on Lille-Brussels with possible connections anywhere in Belgium.
Finally, for TGVs in France, there is an offer (Frequence or Forfait) and you can simulate the price of such a pass. Unfortunately, there is no offer for Lille-Bruxelles nor for Lille-London. To answer @AakashM comment, TGV Forfait pass that offers unlimited rides on TGVs requires an extra fee for every seat reservation (1.5 euro per trip).
On some help page, there is a suggestion to go to a station in Lille to know more about Lille-Bruxelles commutes. I would recommmend to do so, because there are probably quite a lot of people interested in such an offer, the train sales agents probably are used to it.