It was possible under Vista (haven't tried 7) to have an image that is sysprepped using /unattend:unattend.xml. You needed the full path, but I liked to put it in Panther, so path to c:\windows\panther\sysprep.xml. Then you mount the image and overwrite the sysprep.xml file, deploy and reboot. Basically, windows only records the filename specified during sysprep. It doesn't actually parse it until the system boots for the first time, so you could replace the file.