I'm feeling that this is a somewhat naive question but I can't seem to figure this out.
I understand that there is a Redfish service on an OS that could be accessed via an IP network via the Redfish host interface. My question is, what if a system does not yet have an OS installed? Is UEFI capable of presenting its own Redfish service without a functional OS? Seems like some diagrams I see suggest the answer is yes... but is there an implementation?
You typically wouldn't find a Redfish implementation in an OS (but it is technically possible to do this). Generally speaking, Redfish is implemented on a dedicated management controller (BMC), and the OS usually has a dedicated network path to the management controller for invoking Redfish requests. This management controller is dedicated hardware in the system, and runs independent of the OS or UEFI.
So, likewise with UEFI, I would not expect a Redfish service to be implemented by UEFI, but rather UEFI can use the same network interface the OS used to its management controller to invoke Redfish requests.