Skipness,
I think airlines and the service they offer will create the propensity, if that makes sense. Now with having neutralised the EI competition, the only serious competitor ex BHD for the passenger who wants to make a connection somewhere is KLM. Hence, they impact on a move would be marginal.
Also considering that probably somewhere around 50% of pax are connecting ( based on comparable stat for EDI and GLA LHR routes) at LHR and going onwards, the Belfast Airport chosen is note massively important.
There are so many reasons why BFS could be more attractive, for operational and ultimately financial reasons. Eg how many weather diversions per year, how many diversions due to late arrivals at BHD, how much more cost effective could it be for BA to be able to later arrival... Hence reduced unit cost, and all of the above before we would start comparing the airports fees to BA..
I have no information on this topic, just offering my views for counter balance. It feels like some reliable posters are mentioning changes in here, I really do hope the more to come topic doesn't mean withdrawl of Aer Lingus from winter...