Shell or Applescript to delay shutdown?
Why not just change your users' behavior on the shutdown process to make this a lot simpler (with minimal effort on scripting/programming the shutdown hook)?


Tell the users not to use the standard shutdown process (from the menu or through the keyboard shortcut) and instead use your backup AppleScript that's transformed into an app called something like SafeShutdown.app
Within the SafeShutdown app script, do the backup and then end it with a  

tell application "Finder"
    shut down
end tell


or with this (will prompt for password)  

do shell script "sudo shutdown -h now" with administrator privileges


or with a

tell application "System Events"
    shut down
end tell