FYI - I've also opened a StackOverflow question for this: https://stackoverflow.com/questions/53187048/kentico-multisite-azure-application-gateway-app-service-mvc-configuration-issue
It'd be nice from someone on the dev team to explain why a 503 error is used for the licensing fail, and how to get around the Application Gateway limitiation in that it does not forward on requests with HTTP status codes >399.