to me it seems like you are confusing a few things.
You do not need to create a service that returns the 503 page. Your router returns the 503 page and your router is a docker image with haproxy in it. You need to customize you router configuration (see here: https://docs.openshift.org/latest/install_config/install/deploy_router.html#deploying-a-customized-haproxy-router ) and you need to learn how to use Haproxy. This could be helpful:
Am 2016-06-02 um 03:06 schrieb Philippe Lafoucrière: