Finally tried to hook up an AM4 chipset to a raspberry pi (someone suggested this on the pi-pci repository a few years ago), and it turns out it mostly just works.
@omegatotal I guess that would work. I haven't had VCore turned on yet, but that can go up to 1.5V, which I think is the highest of the high power rails on the socket. For anything that's not a compute module (and for those as well tbh), it's easier though to just take 5V from the power supply directly, or through one of the rear USB ports that don't work anyways.
For a full carrier board it would be nice though if it's doable with the thickness of the inductors you'd need.