This is an old thread, but I would like to add a possible suggestion. Could this be implemented by allowing the user to enter their own tags to be extracted (if they exist) from any media files being scanned? Then those tags could also be available in a live list.
Yes, this means more memory, but that would be by user choice.
I have a smart playlist in iTunes that includes a rule based on certain text not existing in the "Comments" tag. I would like to be able to emulate that rule in a Live List, but it is not currently possible.