How can I automatically log out the guest account after a period of inactivity?
Maybe you could consider using SleepWatcher instead of the Security.prefPane; it´s a unix utility you could use to force a logout (or anything else via AppleScript) after a certain period of inactivity.
You can use a commercial product called Power Manager to log out a user after a period of inactivity.
Power Manager will warn the user before starting the log out. During log out any open applications will be quit; applications attempting to block log out will be force quit after a short delay.
Disclosure: I work for the company who make Power Manager.