AA LHR-JFK via KEF ...? I wouldnt have thought so, who is going to want to go on that without significant discount
If I had a day or two to spare, I'd be up for a stopover in Iceland, and given that Icelandair make a fair go at plugging a transatlantic stopover in their marketing, I suspect quite a lot of other leisure passengers who are not too time poor are up for it as well.