I thought that blue screen BOOTROM had something to do with the ability to read/write the NAND by the hardmod's card reader. As to say there has to be a rudimentary software running to access the NAND by hardmod.
Thinking over it that blue screen might also be just a side effect when the card reader accesses the NAND pins, perhaps simply blocking the system from accessing them.
...I remember such a blue screen before the card reader was even connected. I shortened the wires and then the system worked.
That might harden this assumption. If so, a hardmod could never be blocked by a firmware update - if these are simply flash chips with no extra programmable controller inside that would allow some extra communication versus what a simple card reader offers.