something like git-for-each-ref's --format for commits?

I'm trying to obtain various information about commits from a script. The --format option to git-for-each-ref would be perfect, except that it only operates on refs, not arbitrary commit objects.

The --pretty=format:... option for git-log, etc has the annoying property of giving you <unknown> instead of an empty string if an item isn't example (say if you use %b on a commit with no body).

Am I missing some obvious solution here?  

Thanks,

Brian