I did not see "Lincoln," and now probably won't. I read Cashill's book "Deconstructing Obama" and can say with a very high degree of confidence that Cashill nailed Ayers as the author of "Dreams." The number of nautical references in Ayers' own work when matched up against "Dreams" has a mind-blowing number of agreeing terms.
Spielberg isn't impressed with Cashill's arguement? I don't believe it. If a subtle dig was inserted in "Lincoln" as you posit may be the case, that just demonstrates that Cashill got under Spielberg's skin.
I wouldn’t be surprised if that scene were a dog whistle written specifically for Obama and Ayers and a few others to appreciate.
And yes, as soon as I saw it my thoughts were 1) I saw what you did there, Mr. Spielberg, and 2) Looks like Cashill has gotten under their skin.