I think they could do that; WW once said that BWI would work without the stopover; MCO would obviously work for the Summer and they'd only have the stop in one direction, but also no cargo.
Still, there's a hell of a difference between settling for 1:1 and going for what the mini-deal should allow, 3:1. Cullen needs to push hard for this; after all EI has been through over the past few years, the EU should not be allowed to mess things up, particularly over something as irrelevant to EI as foreign o'ship of US carriers. They need to play hard ball too; Bertie has considerable clout at EU level, having concluded the Constitution deal during our last presidency, so he has a role to play. It's not just EI's interests; it's ours as a whole which will be undermined, so if the EU wants its next Constitution to get anywhere (bearing in mind it needs a referendum in Ireland), "don't mess EI about" should be the message coming from Merrion St.