nginx.conf

# ...

  location @app {
    # ...
  
    if (-f $document_root/system/maintenance.html) {
      return 503;
    }
  
    # proxy_pass ...
  }
  
  error_page 503 @503;
  location @503 {
    # intercept 405's so we capture POSTs
    error_page 405 = /system/maintenance.html;
  
    # only use maintenance if it's there
    if (-f $document_root/system/maintenance.html) {
      rewrite ^(.*)$ /system/maintenance.html break;
    }
  
    # else use the regular 500
    rewrite ^(.*)$ /500.html break;
  }

# ...