How can I re-run the initial Setup Assistant on Mountain Lion?
Actually, you should be able to do it by removing a hidden system file which tells OS X that setup has run. Delete this file:

/var/db/.AppleSetupDone


It's a hidden file, so you will have to remove it using Terminal, or show hidden files in Finder. You can remove the file like so:

rm /var/db/.AppleSetupDone


To show hidden files in Finder, type the following into the Terminal:

defaults write com.apple.Finder AppleShowAllFiles -bool true


…and then restart Finder.