Login via HTTP post - site authentication

I am trying to set up Drupal to allow Google's AdSense crawler to view pages via the site authentication options.

My current method is to get Google to send login info via HTTP post to /user/login.

These are the elements I have set Google to post...
[name] => xxx
[pass] => xxx
[form_build_id] => xxx
[form_id] => user_login
[op] => Log in

... but no luck. Does Drupal not allow external log ins? I've been following the advice on this comment: http://drupal.org/node/159574#comment-634583 but it isn't working for Drupal 6.

Any thoughts?