Julie Hilden has a column up at FindLaw about the case. I think it's fair to say that there are many points on which we disagree, and on which reasonable people might; nor have I read the Lexicon to form any opinion about the extent to which it transforms the underlying works. But the thing she has flat wrong is the idea that the Framers would be on Rowling's side because they wished to protect original authors.
Setting aside the minimal constitutional history we have that focuses on the Copyright Clause, such that statements about the Framers are merely our own preferences wrapped in white wigs; setting aside the formalities that made securing copyright very difficult; setting aside the national favoritism that made securing U.S. copyright for an English author like Rowling essentially impossible; setting aside the Founders' copyright term -- there's still the problem of the derivative works right, insofar as it didn't exist at the Founding. In the mid-19th century, the courts ruled against Harriet Beecher Stowe's copyright claim against a translation of Uncle Tom's Cabin into German, because that wasn't close enough to the English work in which she had a valid copyright. Copyright was a right against copying, not a right against changing. The fair use doctrine developed out of abridgement cases, long after the Founding, but nobody argues that the Lexicon is an abridgement. As a shift from the genre of fiction to the nonfiction purpose of cataloging, it is well outside of anything that lawyers in the Founding Era would have recognized as infringement.
There are some good reasons to have a derivative works right for resource-intensive changes in media, as for the movie version of a novel and vice versa. But they weren't known to the Framers.
But, of course, this isn't really about originalism. This is about the right copyright policy now. And that's why the fact that the Framers would have found Rowling's claim nonsensical isn't really important.