DC9_10
I am guessing that what you says makes sense, though I cannot see the driver of a Belfast route for SN being the ability to connect passengers via a United BFS flight, clearly this is the case since BHD was chosen. Moreover, like KLM to Amsterdam, the flights are late enough, so largely designed for point to point traffic. I imagine Brussels Airlines would also far prefer fly passengers directly on their own US services or through their parent Lufthansa in Germany...
EI-BUD