I'm fairly certain that any simple MCU with static clock (ie: supports 0.01 Hz clock, or 100seconds per clock and slower) will suffice as a human level bootstra

I'm fairly certain that any simple MCU with static clock (ie: supports 0.01 Hz clock, or 100seconds per clock and slower) will suffice as a human level bootstrap. ATmega8 happens to satisfy those requirements. It's also not too hard to program microcontrollers that would support human level inputs. In practice, the bootstrap device will be like a Windows7 laptop we've got squirreled away somewhere. It's probably cheaper to buy up a ton of used / old laptops that are sufficient for bootstrapping rather than trying to build methodologies that bootstrap from scratch.