Hmm, I've seen mechanical keyboards with N key rollover available during POST. The only difference is that they were PS/2 keyboards. In fact the n key rollover is said to require PS/2 and USB is not advised.
It's the first time I hear that an USB NKRO keyboard doesn't respond during post because of a motherboard issue. I always thought that was a limitation of the USB stack, not a flaw.
NOX COOLBAY Side Window Black; NOX Apex 600w Modular; ASUS P7P55D-E Socket 1156, Sata 6Gb/s & USB 3.0;
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Logitech K120 Keyboard and Logitech Performance Mouse MX