Passing original commit sha1(s) to prepare-commit-msg hook

Hi,

I would like to use a prepare-commit-msg hook to automatically edit commit comments when a rebase is done, using information from the original commit. For example:

- Adding a commit message line indicating that the commit is a rebase of
  the original commit(s),
- Adding comments based on information stored in git notes on the original
  commit(s).

Currently (I'm using git 1.7.1) the old->new commit sha1 information is passed to the post-rewrite hook; however by then it is too late to do anything with commit messages. During "git rebase" the applypatch-msg hook is called with the original commit sha1 in .git/rebase-apply/original-commit, and during "git rebase -i" the prepare-commit-msg hook is called with current status of the rebase available in .git/rebase-merge/done; however relying on this would seem very fragile to me.

Would there be support for the idea of making the original commit sha1 information available to commit message hooks ? And if so, any advice how I should go about implementing this ? (I have little experience with git codebase).

Thanks,