Ah, in that case, I'm not sure where to point you from the UPS that support ESXi natively perspective. Usually, what I see is that the guest VMs will support a UPS via USB (or USB to serial) and you can assign the USB port to the VM. So in this case, you could assign the USB port to the FreeNAS guest and the use NUT to have any other guest VMs shut down when the UPS's battery gets low. That's about as much as I know about that though.