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.