Hi Urja,
I've just seen that 128MBit serial flash memories with SPI interface (like M25P128) seem to exist. I don't know if/when devices of this type will find their way into motherboards, but these devices would already today reach the limits of serial-flash-protocol.txt (24 bit address and length fields).
Could you add a reserved (and for the foreseeable future ignored) byte to these fields or directly use 32 bit?
Greetings, Frieder