Sounds like a mod request. You should be able to make a small change ("closed ASC ,") to the query (in viewforum.php) that fetches the topics (though I haven't tried it):
// Fetch list of topics to display on this page
switch ($db_type)
{
case 'mysql':
case 'mysqli':
$sql = 'SELECT p.poster_id AS has_posted, t.id, t.subject, t.poster, t.posted, t.last_post, t.last_post_id, t.last_poster, t.num_views, t.num_replies, t.closed, t.sticky, t.moved_to FROM '.$db->prefix.'topics AS t LEFT JOIN '.$db->prefix.'posts AS p ON t.id=p.topic_id WHERE t.forum_id='.$id.' GROUP BY t.id ORDER BY sticky DESC, closed ASC, '.(($cur_forum['sort_by'] == '1') ? 'posted' : 'last_post').' DESC LIMIT '.$start_from.', '.$pun_user['disp_topics'];
break;
case 'sqlite':
$sql = 'SELECT p.poster_id AS has_posted, t.id, t.subject, t.poster, t.posted, t.last_post, t.last_post_id, t.last_poster, t.num_views, t.num_replies, t.closed, t.sticky, t.moved_to FROM '.$db->prefix.'topics AS t LEFT JOIN '.$db->prefix.'posts AS p ON t.id=p.topic_id WHERE t.id IN(SELECT id FROM '.$db->prefix.'topics WHERE forum_id='.$id.' ORDER BY sticky DESC, closed ASC, '.(($cur_forum['sort_by'] == '1') ? 'posted' : 'last_post').' DESC LIMIT '.$start_from.', '.$pun_user['disp_topics'].') GROUP BY t.id ORDER BY t.sticky DESC, t.last_post DESC';
break;
default:
$sql = 'SELECT p.poster_id AS has_posted, t.id, t.subject, t.poster, t.posted, t.last_post, t.last_post_id, t.last_poster, t.num_views, t.num_replies, t.closed, t.sticky, t.moved_to FROM '.$db->prefix.'topics AS t LEFT JOIN '.$db->prefix.'posts AS p ON t.id=p.topic_id WHERE t.forum_id='.$id.' GROUP BY t.id, t.subject, t.poster, t.posted, t.last_post, t.last_post_id, t.last_poster, t.num_views, t.num_replies, t.closed, t.sticky, t.moved_to, p.poster_id ORDER BY sticky DESC, closed ASC, '.(($cur_forum['sort_by'] == '1') ? 'posted' : 'last_post').' DESC LIMIT '.$start_from.', '.$pun_user['disp_topics'];
break;
}