My boot rom will be placed onto 2 Adafruit FRAM SPI breakouts. That will give me 2x 256k external non-volatile memory. This will also allow the system to run on FPGAs with limited block ram.

FRAM works at 40mhz SPI speed, works in the same way as normal ram, but is non-volatile like eeprom or flash but without the delays and paging. So I will be able to update the rom code directly and also execute in place.

Adafruit 2mb FRAM breakout

Both breakouts soldered and working with my Metro M4. Just need to write the hdl now.