When instance is restarting no process is active, varnish cache reply a dirty 503 page.
I would like to be able to customize this page. However, the varnish is shared with other instances. So this is not possible to customize 503 when instance reboot.
Using reloadapplication.php the instance is not rebooting, just the node process.
In this case, the apache process of the instance could reply its own 503 page (specific to the instance).
The only thing missing is a way to configure the error page.
Would it be possible to make a hook for Apache custom error pages ? I mean for example a default error.html page with writing access.