On September 19 2005, I posted a couple of scripts to the monotone-devel mailing list for backing up a Maildir with monotone. It only took 11.5 months, but I now have a public monotone repository up, so I can actually share the script in a sane manner.
The scripts can be found on the org.mudshark.jack.proj.backup_mail branch on the mudshark.org monotone server. See here for more information.