unattend.xml - prompt for machine name

When I ran XP builds I left the 'Computername' line out of sysprep. That way when the machine rebooted and went into setup it would prompt me to enter the computername. I'm looking to do this with windows 7 but so far had no success with the unattend.xml file. anyone got this going ?/able to point me in the right direction ?

Thanks.