The main problem seems to be that the only way the BIOS can boot an OS is a mass storage device with executable code in the boot sector, in contrast to older computers that had some sort of monitor/debugger in their ROM. I haven't researched disk drive interfaces, but trying to hook up a floppy drive to some toggle switches so I could write a boot disk would be my first step. But then modern motherboards don't have floppy drive controllers ¯\_(ツ)_/¯
Of course you can also try to reprogram the BIOS ROM on the motherboard, but you're likely to need a full OS to first reverse engineer it enough to know where to stick your debugger code.