[13]

I'll have to find the data sheet, but it should be very possible to pull a pin high or low to disable writing to the chip itself. I doubt they do any checks for being able to write to the chip in software, that would just wear out the flash unnecessarily.

[bravo22]

SPI flash has a write disable pin but that would also prevent Apple updates.

[13]

Could bodge in a physical switch for write protect.