IPMI provided a "System Firmware Progress" sensor. It provided things like "Memory Initialization", "Secondary Processor(s) Initialization", ...
IPMI also had a "Base OS Boot / Installation Status" sensor. It had things like "boot completed" and installation status info.
On our POWER based servers, we cobbled these together to be able to show our users a status of the boot of their system, from the BIOS, to our Hypervisor, up into the OS.
From what I see with Redfish, we basically have the State enum (http://redfish.dmtf.org/schemas/v1/Resource.json#/definitions/State) associated with the ComputerSystem, Manager, and Chassis Schemas. This provides fairly limited status compared to the detail we had with the IPMI sensors.
I'm wondering if there has been any discussions on how to enhance the status provided to a user of a boot of their system? For us we'd want to show a few different states as the BIOS boots. A few more for the Hypervisor. Then probably a few more for the OS.