I skipped pwrstatd, as I had already done enough research on NUT to know that it was the direction I wanted to go. In particular, I really like having notifications for power outages.
If you haven’t already done so, I previously wrote a guide to add a manual shutdown/reboot switch in OH. It uses pretty much the same sudo permissions that you implemented for pwrstatd, except that my concern was only controlling the host RPi (not a client). Sorry I didn’t realize that sooner!
If someone wanted to have OH shut itself down during a power outage, they’d just need a rule to trigger when the UPS’s battery runtime or status falls below a certain threshold and run the shutdown command. I left that out of the guide, since I think it’s better to just let NUT do it.