arduino-based Redfish server implementation

Can't say I've seen a solution for this, but I will at least say OpenBMC is fairly mature at this point with their Redfish implementation. You're right in this is typically something you see on a dedicated BMC on a motherboard, but it does allow for the flexibility to deploy it on other devices. Unfortunately retro-fitting something like this into a desktop system is a bit beyond me, and I'm not sure if it's very feasible since there's usually tight coupling between the BMC and hardware design to give it the necessary hooks to perform out-of-band management operations.

For what it's worth, if you're able to keep the systems running at all times, you could have a Redfish service running directly on the desktop system, and thus avoid any hardware modifications. There is a Python-based service here that you could extend as you see fit: github.com/DMTF/Redfish-Interface-Emulator. But you'd lose the "lights out" aspects by going this route.