would it be economically viable to open a 1 aircraft base.
I wouldn't know the economics of it but plenty of airlines station aircraft at airports that are not bases eg Bmi at the city, I know they normally have 1 aircraft based there although they do have 2 this summer. However the low cost airlines seem to night stop only at their dedicated bases. If EI operate Belfast-LHR then they should definately night stop at Belfast.