Okay, I've got it figured out.
Since the script is being executed by CopyJob instead of SuperDuper, delays only affect CopyJob. So I told SuperDuper to delay 5 minutes. Here's the code:
Code:
on beforeRunningCopy()
tell application "Finder"
mount volume "smb://192.168.n.n/BACKUP_VOLUME"
open file "BACKUP_VOLUME:Backup.dmg"
end tell
tell application "SuperDuper"
delay 300
end tell
end beforeRunningCopy
It'd be nice if SD would make automatic backups to regular .dmg files on a server without all this fooling around... why doesn't it?