A number of years ago I created a custom PHP script for generating a podcast XML file for a Podcast i had submitted for inclusion into iTunes.
At the time, i'd managed to find a document detailing the various iTunes custom tags. Fast forward 4+ years, and i now cannot find any of it in one place online, not even from Apple.
All i could find was references to changes to the iTunes Podcast file spec, but again these only refer to changes/additions to tags.
Just to point out, i've tried web searches, but thus far have only found the odd thing here or there.
The documentation for the iTunes specific tags can be found here.