Hi,
You can try to use
https://github.com/coreboot/bios_extract/blob/master/change-mcp55-mac.pl
But! First you need to change it accroding to romstrap tables position see
coreboot/src/southbridge/nvidia/mcp55
See romstrap.inc/romstrap.ld
Question is if the legacy BIOS has a checksum of bootblock or not...
Or, you can try to find the MAC in the image and then change it and see if the machine still boots...
Alternatively you can check dumps from 2 machines to see where the checksum is...
Thats all I can do for you
Thanks Rudolf