Instead of using capistrano's deploy:web:disable (http://capitate.rubyforge.org/recipes/deploy-web.html#deploy:web:disable), why not just have the maintenance page displayed anytime the upstream server isn't available?

app_vhost

...

  try_files $uri.html $uri @app;
  error_page 502 503 =503  @maintenance;

  location @app {
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header Host $http_host;
    proxy_redirect off;

    proxy_pass http://app_server;
  }

  location @maintenance {
    try_files /503.html =503;
  }

...