Amos at 2011-07-21 14:12:20:
1) Everyone should receive some form of credit for directly contributing to a movie. I recall Clooney giving Special Thanks credits to actors whose scenes were cut from Confessions of a Dangerous Mind (class move). Even if writing credits, for good reason, hold specific meaning, I definitely think something like what you said or "additional material by" is fair and appropriate.
2) Have you told the story of using a pseudonym? Could you point us to it or elaborate?