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