Since the frontend code for a comments plugin is already publicly viewable, it only makes sense for it be fully open source on github. Big sites would then have to review and pull the latest changes and host them on their domains.
Technically:
By default, a blog could be set to load the comments iframe from a big site like facebook or google. If the person was not logged into that domain, instead of being prompted to login, they would be redirected to google, and so on and so forth until a domain just isn't being found. Only then would a user be asked to login with one of the services.
Also, since this would be pretty magical/mystical to a user it would be important to show "Logged in through facebook.com" with the option to login through a different domain (or maybe connect/merge accounts)