CPA periodically checks the weather condition (Weatherfeed) and takes action on "Unsafe".
I had hoped that CPA could do something similar, that is parallel to the Weatherfeed check a Digital-In that signals LossOfPower and run the ShutdownObs on LOP.
Unfortunately vain hope.

I'm thinking of two (maybe three) different ways to proceed:
*** run a little program separate from CPA that, at intervals, interrogates such Digital_In and upon LOP runs a script as outlined by BOB a few days ago. This script should stop current actions in CPA and start the ShutdownObs.
*** As a totally different approach, a bit based on Peter's idea of "fooling" CPA, I'm thinking of a hardware-only solution.
Have a relay or something similar that upon LOP switches on a little LED above the WeatherStation. The WeatherStation interpretes this as a bright sky and responds with an "Unsafe" condition. CPA will do the rest.

A little test with a flashlight gave the hoped-for response.
Another nice aspect would be that, when the grid power is restored, the LED would be switched off and the weatherstation would report Safe again. CPA could continue.
*** More or less as an afterthought: why not increase the power-backup capacity so that the observatory can run all night without grid power.