Since November, passengers on the Eurostar from Brussels to other Schengen destinations (Lille or Calais) now checkin and clear security in a different area.
From the Eurostar conditions of carriage
Check-in at Brussels-Midi for your journey to Lille or Calais will take place at the intra-Schengen Terminal. This dedicated check-in area is located on the mezzanine floor of Midi station accessible by the stairs and lifts rather than in the Channel Terminal.
If you have a suitable Railteam loyalty card which gives you lounge access, are you able to access the Eurostar Business Premier lounge on the ground floor, having gone through the Schengen checkin area on the mezzanine floor? Or are intra-Schengen passengers kept upstairs after checkin, unable to get down to the mezzanine to access the lounge?
(It appears from this question that you can't buy a Business Premier ticket for intra-Schengen travel, so you won't get ticket based lounge access, only Railteam Loyalty Card based access could be applying)