As I mentioned earlier, I've now got all these Apple Lossless files, finally ripped from those DVDs and ready to integrate with my iTunes library. The hard part is going to be metadata.

I can certainly import the new files into iTunes and remove duplicates with something like the built-in duplicate finder, iDupe or Corral iTunes Dupes, but that's only half of the problem. On many of my MP3 files, I've put a lot of energy into the metadata, including BPM, star ratings, and consistent spellings of band and song names. I haven't been able to find any tools that can do the tag migration that I need. Any suggestions? Hack one of the above dupe tools?

I think I'd look into editing the iTunes Music Library.xml file directly. It's an easy to understand format.

Editing the XML is relatively straightforward, but I also want to get the tags, inside the files, to have the same metadata updates. For MP3, there are all kinds of libraries. Anybody know how Apple does tags in its AAC or Lossless files?

I'm not totally sure about this, but how I've observed iTunes work in regards to tagging is:

1) If file is added to library, the XML file is updated to contain the meta data. This is the ONLY TIME it is read from file.
2) When you update tags in iTunes, it updates the XML and the file meta simultaneously, if there is a discrepancy, with what's in memory.

Since the XML is already populated with everything in your library, editing the XML file before loading itunes should show the edited information. So theoretically, at this point you should be able to select all, get info, make some trivial change (like tick volume djustment on but leave it at 0db) and hit OK. It will take a while because all the files are being checked, but will result in what you want.

I never tried this because I never needed to, but if my first assumption is correct, then the second should work.