I was looking into this recently. It doesn’t seem like you can easily get your hands on BMC hardware. There is one project I found where they’re using an FPGA and everything is open source but it still looked far from easy.
Afaik the problem is typically not hardware but software, since most server motherboards come with a bmc chip on it. Are you designing a motherboard? All i would ask for a bmc is root access. But now everyone (but the big players, who design their own motherboards) is basically stuck with a stupid embedded linux with shitty software that has half-backed features they don't even need, but now need to care about. When all you'd need would be a way to access a host serial console, read sensors, control boot sequence and perhaps re-flash the bios, ie most likely just basic interactions on serial interfaces.
> Are you designing a motherboard?
Yes, the team I work for designs server motherboards.
BMC security is what keeps me up at night. Firmware software quality is low, and often not up to date. I think openbmc does a good job in both respects.