Sort on Custom Field presence, date modified

// Sort posts created/modified in last 24 hrs in order on presence of a custom field,
// then date published.
$meta_key = 'featured';  // The meta_key

function mam_posts_fields ($fields) {
   global $mam_global_fields;
   // Make sure there is a leading comma
   if ($mam_global_fields) $fields .= (preg_match('/^(\s+)?,/',$mam_global_fields)) ? $mam_global_fields : ", $mam_global_fields";
   return $fields;
}
function mam_posts_join ($join) {
   global $mam_global_join;
   if ($mam_global_join) $join .= " $mam_global_join";
   return $join;
}
function mam_posts_where ($where) {
   global $mam_global_where;
   if ($mam_global_where) $where .= " $mam_global_where";
   return $where;
}
function mam_posts_orderby ($orderby) {
   global $mam_global_orderby;
   if ($mam_global_orderby) $orderby = $mam_global_orderby;
   return $orderby;
}
add_filter('posts_fields','mam_posts_fields');
add_filter('posts_join','mam_posts_join');
add_filter('posts_where','mam_posts_where');
add_filter('posts_orderby','mam_posts_orderby');

global $wpdb,$wp_query;
$now = date('Y-m-d',(time() - 60*60*24));

$mam_global_fields = ', IF(ISNULL(pm.meta_id),9,1) as sort_key';
$mam_global_join = " LEFT JOIN $wpdb->postmeta pm ON ($wpdb->posts.ID = pm.post_id AND pm.meta_key = '$meta_key')";
$mam_global_where = " AND $wpdb->posts.post_modified > '$now' ";
$mam_global_orderby = "sort_key, $wpdb->posts.post_date DESC";
query_posts('posts_per_page=-1&ignore_sticky_posts=1');
//echo 'REQUEST: ';print_r($wp_query->request);
if (have_posts()) {
   while (have_posts()) {
      the_post();
           echo "<p>"; the_title(); echo ' '; the_time('Y-m-d');  echo " $post->sort_key</p>";
   }
}