add_filter('the_title', 'custom_the_title');

function custom_the_title($title){
    global $page_id, $post;

    // Do not apply this filter if the post is not in the_loop
    //  --- this excludes nav menus
    if( !in_the_loop() )
        return $title;

    if ($page_id)
    {
        $custom_title = get_post_meta($page_id, "custom_title", true);
    } else {
        $custom_title = get_post_meta($post->ID, "custom_title", true);
    }

    //  Check if there is a metadata for title
    if ($custom_title)
    {
       //  Do stuff here....
    }
    return $title;
}