In general it's best practice to leave unused ports on managed switches in an admin down/shut state until something you know is connected. Or live, but in a quarantine VLAN.
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.
My college used to do similar. If you did not register your MAC address, you would be DHCP assigned into a walled-garden IP block.
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'.
Out of curiosity, you couldn’t just guess them based on knowing a couple? Or do people assigning them in some fashion that isn’t consecutive within the block?
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.