未经测试，但应满足以下条件：

add_action( \'posts_clauses\', \'wpse110779_filter_admin_search\', 10, 2 );
function wpse110779_filter_admin_search( $pieces, $query ){
      global $wpdb;

     //Check if this is the main query,and is a search on the admin screen
     if( $query->is_search() & $query->is_admin() && $query->is_main_query() ){

          //Collect post types & search term
          $post_types = $query->get(\'post_type\');
          $search = $query->get(\'s\');

          //Check if query is for \'my-cpt\' post type
          if( \'my-cpt\' == $post_types 
               || ( is_array(  $post_types ) && in_array( \'my-cpt\',  $post_types ) )
          ){

             //Set up meta query
             $meta_query = array( array(
                \'key\' => \'my_key\',
                \'value\' => $search,
                \'compare\' => \'LIKE\'
            ));

             //Generate sql
             $meta_sql = get_meta_sql( $meta_query, \'post\', $wpdb->posts, \'ID\', $query );

             $pieces[\'join\'] . = " ". $meta_sql[\'join\'];
             $pieces[\'where\'] . = " OR (" . $meta_sql[\'where\'] .")";
          }
     }
     return $pieces;
}