Some motherboards just have a physical jumper that prevents BIOS flashing. This happens infrequently enough as to warrant it for one server, or 10 servers, or maybe 100 servers. Likely unpractical for 1000 servers though.
Make this one simple enough and add an EPROM for it. Effectively a security chip for the oob. Extra points for secure enclave-like verified boot.
> Often we see.. great security.. compromised by other great ideas for mgmt and other things.. starts to weaken its security posture.. want to keep Caliptra very clean [via OSS firmware transparency]