The page must contain a form with an action field set to /login/index.php
and return fields username and password
It may be worth checking the URL when it's entered at admin/auth.php
to ensure it is at least pointing to a valid page as an incorrect URL
could potentially lock all users out of a site.
// auth.php - created with Moodle 1.5 UNSTABLE DEVELOPMENT (2005010100)
+$string['alternatelogin'] ='If you enter a URL here, it will be used as the login page for this site. The page should contain a form which has the action property set to <strong>\'$a\'</strong> and return fields <strong>username</strong> and <strong>password</strong>.<br />Be careful not to enter an incorrect URL as you may lock yourself out of this site.<br />Leave this setting blank to use the default login page.';