Brussels wasn't a very important town during the Lotharingian Era (later dukes of Lothier & Brabant did claim to be the rightful heirs of Lotharingia) and it was the good relation between their lord, the duke of Lothier & Brabant and count of Leuven/Louvain & Brussels, and the town Brussels, which led to stimulation of the development of Brussels.
So even with West Francia more focused on Lotharingia, it will be uncertain if Brussels will be the capital; maybe Aachen/Aix-La-Chapelle/Aken will be the capital, after all that was the town of Charlemagne.