Having designed this, the next step is to get boards made at OSHPark and try it out. Then I have to figure out how to actually get an open-collector output from PiKVM. I have no doubt it's possible, I just haven't yet explored exactly how to do it in the PiKVM UI. I might reach out on the PiKVM discord channel to see if someone can help with that.

Also, note that if you drop power to most modern macs, they won't necessarily turn back on when the power is restored. There are two ways around this: 

In Settings > Energy (on a non-laptop Mac) there are two switches you can (and should) turn on. One is "wake for network access." This allows Ethernet Wake-on-LAN packets to power up the mac. The other is "Start up automatically after a power failure," which is self-explanatory.