A head up for the distributed gits and python users:
I just pushed a commit to cworth/master implementing
Message().get_filenames(). This will return a generator of absolute
filenames for a certain Message-ID.

For further information I refer to
http://git.notmuchmail.org/git/notmuch/commit/b31247c354b54a3cbeb1c7f9df830e16f7c921d9
and the code (and it's documentation).

Let me know if it works for you.

Sebastian