I could, yes, go through Drupal's code to see how the login session is handled, but I was hoping someone had already tackled the problem previously.
As for the open authentication system, the actual login could be performed on dplogin.com, but the credentials could be passed back to the site needing the login. That way, dplogin.com will not have to trust a third party, players will not have to trust a third party, and the third party will not have to trust the player. Nobody loses, other than for some inconvenience. For more convoluted login procedures, they could be allowed through a different API on a case by case basis. One thing to note about this system is that it requires an elevated sense of awareness on the player's part; the player must ensure that the login page is at the right URL, and not at a falsified one.