BACnet already exists as an ISO standard: https://www.iso.org/standard/37298.html https://bacnetinternational.org/
The example application in the Ethernet Alliance one-pager on single pair Ethernet is HVAC: https://ethernetalliance.org/wp-content/uploads/2020/10/EA_T...
This could be prototyped on a BeaglePlay :)
BACnet allows multiple PHY implementations, including 802.3
I suspect that 802.3.cg is also kosher, but haven't looked in detail. It's been 5 years since I looked seriously at whether BACnet was the correct solution for a product family.
Single pair Ethernet is indeed cool tech.