<p>I'm looking for an open-source library for editing AAC metadata in M4A files (as appearing in iTunes).  Specifically, I need to be able to add an album art tag.  It should be something similar to id3lib, which works with ID3 tags on MP3 files.</p>

Since I hadn't find any open-source library maybe open-source program, that can read and set iTunes-style metadata tags in MPEG-4 files, will help you.
atomicparsley.sourceforge.net
There is also Known iTunes Metadata structure provided:
http://atomicparsley.sourceforge.net/mpeg-4files.html
This discussion may also turn out helpful for you:
www.hydrogenaudio.org/forums/lofiversion/index.php/t31022.html
Good Luck