I have a dumb custom webserver behind nginx (that i am stuck with), that will occasionally go into a maintenance mode, where it is not able to serve its' normal content, but instead just sends "File not found" message in response body with HTTP code 200.

Unfortunately at the moment there is nothing I can do to make the upstream webserver to behave and send a 503, which I could then handle. Is there any way I can match against such upstream output to redirect it to a 503 page served by nginx?