I really don't understand why sterile non-Schengen arrivals (e.g. ex-UK) can't be landed into the B-pier departure area, rather than having to go through security again. That's the standard procedure at MUC / ZRH etc. It's ridiculous to do the short hop from from BRS-BRU and then have to trek through unncessary security again at BRU, before either going upstairs for another non-Schengen destination, or through transfer immigration and into the A-pier
I did email BRU airport this question, and never got an answer, but then given the relentless mediocity of BRU, I wasn't terribly surprised about that.