I got JHymn set up and running to perform almost exactly the same action as my M4P Backup script. The difference is that my backup script created the unlocked M4A file in the same folder as the original M4P, then copied both the M4A and M4P into the backup location; the configuration of JHymn that I’m using just creates the M4A in the backup location - it doesn’t back up the M4P, nor does it place a copy of the M4A alongside the original M4P. Other configurations of JHymn can do things like replace the original M4P with the decoded M4A… but I didn’t want to go that route.
Anyway, here’s my JHymn configuration. I’ve upgraded to iTunes 4.7.1 and everything works fine. Click the screen shots to get a larger view.