function mycpt_rewrite() {
    global $wp_rewrite;
    $queryarg = 'post_type=mycpt&p=';
    $wp_rewrite->add_rewrite_tag('%mycpt_id%', '([^/]+)', $queryarg);
    $wp_rewrite->add_permastruct('mycpt', '%mycpt_id%/%post_name%', false);
}
add_action('init', 'mycpt_rewrite');



add_filter('post_type_link', 'mycpt_permalink', 1, 3);
function mycpt_permalink($post_link, $id = 0, $leavename) {
    global $wp_rewrite;
    $post = &get_post($id);
    if ( is_wp_error( $post ) ) {
        return $post;
    }
    $newlink = $wp_rewrite->get_extra_permastruct('mycpt');
    $newlink = str_replace("%mycpt_name%", $post->post_name, $newlink);
    $newlink = str_replace("%mycpt_id%", $post->ID, $newlink);
    $newlink = home_url(user_trailingslashit($newlink));

    return $newlink;
}