Even if such a direct link is present, it will lead to the starting forums page in the case of $enableNewRegistrations=FALSE,
Unfortunately that is not the case. The link is the same override you gave in your manual to enable the admin to register someone, and it goes not to the main page but to the "confirm acceptance of the rules"-page of the registration process.
The code (copied directly from the eng.php file) is
$l_anonDisallowed='Only registered users are allowed to post here. Please enter your login/password details upon posting a message, or <a href="'.$main_url.'/'.$indexphp.'action=registernew" target="_blank">sign up first</a>.';