@drscriptt @cks This might be a useful starting place, giving an example of connections between a BMC and the rest of a server-grade system. https://www.issi.com/US/newsletter/Issue86_June_2021/ISSI_BMC-Engangement.pdf
At first, I was assuming that at least part of the PCH or equivalent would need to be powered-up for remote management, to allow for PCIe communication between the BMC and the management NIC, but maybe the RGMII connection eliminates that need? Maybe that's only for machines with a dedicated management NIC? Anyway, I've ended-up with more questions than answers after trying to dive-in to this topic.