Si queremos modificar las búsquedas que ofrece WordPress para que devuelva lo que nosotros queremos tan solo hay que hacer dos funciones para dos filtros:

add_filter('posts_results', 'mi_search');
function mi_search($posts) {
  $s = get_query_var('s');
  if (condicion($s)) {
    $posts = array();
    $paged = intval(get_query_var('paged'));
    if ($paged < 1) $paged = 1;
    $posts_per_page = intval(get_query_var('posts_per_page'));
    $from = ($paged-1)*$posts_per_page;
    global $wpdb;
    $_posts = $wpdb->get_results("select post_id from $wpdb->posts where loquesea order by post_id desc limit $from, $posts_per_page");
    foreach($_posts as $p) {
      $posts[] = get_post($p->post_id);
    }
  }
  return $posts;
}

add_filter('found_posts', 'mi_found_posts');
function mi_found_posts($n) {
  $s = get_query_var('s');
  if (condicion($s)) {
    global $wpdb;
    $res = $wpdb->get_results("select count(*) as n from $wpdb->posts where condicion ");
    $n = $res[0]->n;
  }

  return $n;
}