as far as i understand building sys-power/nut with useflag zeroconf is supposed to activate avahi support. This is done by copying the created
./scripts/avahi/nut.service to /etc/avahi/services (that's done by Ubuntu).
That's not happing on Gentoo so nut-scanner --avahi_scan (handy thing) isnt working.
Simple fix would be to extend the ebuild to install the nut.service when useflag zeroconf is enabled.