After using ProcMon for a few minutes it seems there is no registry entry containing update preferences, it appears to be stored in a proprietary format in a file called QuickTime.qtp (QuickTime Preferences file).
It is located at:
C:\Documents and Settings\[username]\Local Settings\Application Data\Apple Computer\QuickTime
where [username] is the name of the user account.
First, turn off automatic updates on a system, copy the QuickTime.qtp file, and during sysprep you could have this file copied over to each computer's directory shown above.
Out of curiosity though, why are you putting yourself through this using sysprep? WDS is far superior in many ways. You can configure a "skeleton" computer just the way you like it, then image it and deploy it to multiple computers.