In the FOG web management, you can update the value under Other Information -> Fog Settings -> FOG_CHANGE_HOSTNAME_EARLY to 0 and the imaging script will not try to update the username by editing a registry key before windows loads.
If you do not install the FOG service, you’re hosts will not rename after they load windows. You can also disable the hostnamechanger option in the FOG Service install, per host record, and for your FOG server completely.