[kh99]

Stealing some code from inlinemod.php to make a plugin using newpost_complete, I found that this:

$forumid = $foruminfo['forumid'];
if ($threadinfo['visible'] AND can_moderate($forumid, 'canopenclose'))
{
	if ($type == 'thread')
             $threadid = $post['threadid'];
         else
             $threadid = $threadinfo['threadid'];

	$vbulletin->db->query_write("
		UPDATE " . TABLE_PREFIX . "thread
		SET open = 0
		WHERE threadid = $threadid
	");

         $modlog[] = array(
			'userid'   =>& $vbulletin->userinfo['userid'],
			'forumid'  =>& $forumid,
			'threadid' => $threadid,
		);
         include("includes/functions_log_error.php");
	log_moderator_action($modlog, 'closed_thread');
}

will close the thread after the post is made, only (I hope) if the user has the right permissions. So if someone can figure out how to tell if the post had the [closethread] tag in it, I think you'd be there. I suppose one way might be to just check for a certain text string at the end of the post instead of using bbcode.