I think Jet chose BRU because they wanted an easy way to transfer pax to other major European cities from just a single European point of arrival by codeshare (which they got in the shape of Brussels Airlines and its extensive network of destinations). MAN at the time could offer neither a single terminal operation or a single airline with sufficient connections to other European cities to codeshare with.
As for AI that I think is a different story, as no one seems to know what are they really looking for? A plain and simple tech stop, an airport that can also provide local arrival/departure footfall, subject to traffic rights or a true hub style operation like Jet in BRU??
If the first then any airport surely would suffice, subject to slots and cost (hence the DUB option). If the second then I think MAN stands a very good chance indeed based on local catchment and airport infrastructure. If the third, it will be somewhere like CPH or MUC where they could interline their pax with SK or LH across Europe - of course subject to a codeshare being agreed in the first place.......