Serial console + reset line control for the win here, if you want easy to diy. I guess there are lots of arm32/arm64 embedded boards that run openbsd you might repurpose and be able to mount inside a machine and which will have near zero power footprint compared to the hardware itself?

I've often wondered about actually replacing the kernel and userspace on the vendor BMCs themselves, to substitute something more competent, but I've not found anyone who's successfully done it.