Nice, thanks for that info! Can you share any pointers on applying phase inversions? I've tried that a couple times but haven't had too much luck!
I agree it sounds like Phil on "Man of Our Times". Reminds me of how he changes the timbre of his voice for some of the verses in "Dodo"
I can't remember exactly how to get the option, but I think there's a link on the wikipedia entry of Audacity to download the ability to import files in ffmpeg format. You need that to import the files from the DVD. Then, you wait for awhile to get all the tracks imported in from the Video folder from the DVD. Finally, you just play around by putting two of the files together and inverting one of them (by selecting "invert" from the Effect menu).
Having said that, I think one of the files on each of the DVDs are the soloed lead vocals. Some of the backing vocals are in there too.
You're right in that Phil changed his timbre a lot in those days; he was brilliant at that!