AFAIK Kazaa has paid content

With ZIP files you'd nee updatable password protection lists inside the ZIP file or a programm that has many generic keys for the same ZIPfile or a programm that is DRM like (online checking for permission) - all which are non existend as far as I know.

BUT.. what if you spread the zipfile along with the password on P2P - pack a few samples into the zipfile and an *.nfo file like the crackers do which points them to the source of the vids/pics.
You could also include a working user/pwd with the "crack" which is only good for a SMALL area of your paysite - then have them upsell.