I have an idea, but not sure how to implement this (and whether it's possible at all):
When Unraid server shuts down\restarts - it sends some API command to Kuma so that it changes some websites to the Maintenance mode. And when back online (and Docker engine started) - send command that Maintenance ended (normal mode).
I have Kuma on an external server (VPS), so it still works when Unraid, hosting services is down