Asus P8H61-I LX R2.0 Default Mac Address after BIOS replacement

[BorderMicro]

Hi
Are there any tools out there to change the default mac address on a replacement BIOS chip for these boards.
We have needed to replace about 40 of these, unfortunately some are on the same network subnet which is causing issues when we need to rebuild them using network boot. I can change the MAC in the OS but this is obviously not retained on a rebuild.
Tried using the AFUDos util but with no joy.
I'm not a BIOS modder so know very little about the internal workings of these.
Can anyone help?
Cheers Duncan