convert Mail.app mailbox to a directory tree of plain text messages

Is there an API or tool to convert a Mail.app mailbox like

~/Library/Mail/V2/Mailboxes/my.mbox to a directory tree of plain text messages, one per file ?

Then I could grep, prune old messages ... in Unix, as opposed to WysiALLyg.

(I realize that even a single "message" may have a complicated structure hard to map to a Unix file tree; but this must have been worked out in the elder days of email.)

(Related: documentation-on-apple-mails-emlx-data-structures-for-conversion-purposes on SO.)