From the datasheets, the K9F1208UOM ID is 76C0, but the K9F1G08UOM's is F115 - the Z would be querying the ID and giving up on the unfamiliar one, even if the chips are functionally identical.
The only real way to get it to work in that case would be to cram in a microcontroller to intercept the new ID and inject the old one in a similar fashion as a console modchip does.
It might also be possible to patch the in-flash version of the firmware to replace the ID with the newer one - though if that worked you couldn't use the Fn+D+M menu (as it's in ROM) to save you from a bad subsequent flashing...