You don't need a mod to do this, you could put a meta refresh tag in the header in an if statement- if the user is a guest forward to the register page...
Code:
<if condition="$show['guest'] AND THIS_SCRIPT != 'register'">
<meta http-equiv="refresh" content="30;url=http://yourdomain.com/forums/register.php">
</if>
That will forward a guest to the register page in 30 seconds, but not if they are already on the register page. You need to give them time to login before forwarding them else people will get annoyed.
Also I'm not sure how this would affect SEO as spiders are guests and may get forwarded too...