$feed = new DOMDocument();
$feed->load('http://www.omnimaga.org/index.php?action=.xml;type=rss2;limit=100');
$feed = $feed->getElementsByTagName('guid')->item(rand(0,99))->nodeValue;
header('Location: ' . substr($feed, 0, strpos($feed, 'msg')) . 'new#new');

Are you able to get the username of who's going to the page(the user)?
Couldn't you try the parse the new posts page for that? I think that might be easier

$feed = new DOMDocument();
$feed->loadHTMLFile('http://www.omnimaga.org/index.php?action=ezportal;sa=page;p=5');
$feed = $feed->getElementById('block74')->getElementsByTagName('tr');

do
{
    $item = $feed->item(rand(0,99))->childNodes->item(2);
}
while (strtolower($item->lastChild->previousSibling->nodeValue) == strtolower($_GET['avoid']));

$item = $item->firstChild->nextSibling->getAttribute('href');
header('Location: ' . substr($item, 0, strpos($item, 'msg')) . 'new#new');

EDIT: Deep Thought while clicking around with it I still see it going to posts that aren't at the bottom. Its going to the first post in this topic http://ourl.ca/11156.new#new

EDIT2: it seems to be doing it on other topics also

Thank you for singling me out for this award Deep Thought . Maybe someone could add a mod to the site that will put this on the forum page or up on the top and auto ad the users name.