I think what you are asking for is what is being discussed in this thread here
That thread was started after a USB-C UPS was created for about 15 people that functions amazing for the HE Hub. It is basically a 5v battery pack that will keep the HE up for hours if power is lost.
That thread talks about taking that device to the next level with integrated Zwave/Zigbee radio to communicate state to the hub so it can be managed better. There is even talk about having a method to restart the hub after power has been restored to the ups.
The one thing that really stands out to me though about that UPS is that when I tested mine it lasted 14.5 hours. If my house is without power for 14 hours I am dealing with much larger issues then my HE environment most likely. It is amazing how little power the hub does sip and how long it can run with even a very modest battery backup.
