Non-interactively rewording commit messages

Hello, all:

What's the best approach to non-interactively rewrite specific commit messages? In this particular case, I am trying to automatically retrieve code review trailers sent to the mailing list and put them into corresponding commits.

For example, I have a set of commits:

abcabc: This commit does foo bcdbcd: This commit does bar cdecde: This commit does baz

They were all sent to the mailing list and a maintainer sent a "Reviewed-by" to the second commit. In a usual interactive rebase session this would be:

pick abcabc reword bcdbcd pick cdecde

When the edit screen comes up for the bcdbcd commit, the author would manually stick the new trailer into the commit message. However, I can automate all that away with b4 -- just need a sane strategy for non-interactively rewriting entire commit messages at arbitrary points in the recent history.

Any pointers?

-Konstantin