"An American carrier at DSA"
I don't doubt the facts regarding the potential interest by Americans in visiting South Yorkshire but unfortunately you have to face facts.
Americans are by and large very badly educated from a geographical standpoint. They know roughly a handful of places in the UK (London, Stratford upon Avon, Edinburgh, and at a push York). Ask them to put any of them on a map and they might get two out of four, assuming they know roughly where London is!
Given that US Travel Agents are probably no more worldly wise than the general population, how exactly is an airline going to get them to book customers to Doncaster ("is it near London?????!!!!").
I think you will find that BHX and MAN already have the same problem, with the majority of T/A traffic being UK originating.
It's a great idea, but I just don't buy it in the short term anyway.