I don’t think it has much to do with OpenID – my suggestion still keeps the classical username/password tuple as credentials, but simply disallows the user to c

I don’t think it has much to do with OpenID – my suggestion still keeps the classical username/password tuple as credentials, but simply disallows the user to choose the password freely. One extension, of course, would be for websites to somehow signal to browser ‘Hey, please store this password/username for this website’, but I’m not sure whether that's necessary.