We have some sites with a single-host XCP-ng pool backed by a small UPS. We install nut directly in dom-0. I'm aware of the policy for adding anything to dom-0 but we believe this usecase fits in the recommendations (simple enough, no vast dependencies, marginal resources usage, no interference ...). With proper testing works pretty well. nut inside a dedicated RPi definitely makes sense for a site with multiple hosts backed by the same UPS.