Tente assim.
error_page 403 http://yourdomain.tld/403page.htm;
location /update.php {
allow <your-server-ip>;
fastcgi_param SCRIPT_FILENAME /srv/www/mysite/public$fastcgi_script_name;
fastcgi_pass 127.0.0.1:9000;
include /etc/nginx/fastcgi_params;
deny all;
}