Couldn't Google just auto-register an account with a known good password, and penalise the account if it fails? Someone else mentioned improving the http authentication 'api', which would definitely help in this regard; until then there are drawbacks to the auto-register approach including things like captchas. Sites would have to explicitly allow the Google bot to bypass them.
How do you check if password is salted and hashed correctly afterwards? There's not much use of strong passwords if they are stored in plaintext anyway.