Imagine a small loop-back-like device which is plugged into all open network ports - if any of them are removed from a network port, an alert is generated stating "device from port 48 on switch 1 in closet 0 was removed"
Your idea, however, is not totally uncommon to have a raspbery pi sized device at an offsite location, specifically not plugged into any sort of UPS, which is monitored by various alerting systems. In addition to the alerts that one should get during a grid power failure event from managed UPS and automated generator transfer switch systems, the disappearance of your "UPS canary" can indicate that something is going on at an unattended site.
We found we could run an IP scanner on the authorized subnet (from a computer with a whitelisted MAC), and find the unused IPs, and just set those statically for 'visitors'.
No need to register any more MAC addresses.
Oh, and all authorized IPs were in a public address space.