After hunting around, I found a piece of free software called Lostify that lets you edit the m4a tags. The only problems remaining are that you need to convert your library to AAC (if you haven't already), and there is still no option in Smart Playlists to filter by content rating. Still it could come in handy.
[robg adds: Lostify is a GUI for the command-line tool AtomicParsley. We mentioned AtomicParsley in this hint (and the comments mention Lostify as well, but I felt it worth sharing as a separate tip.]

