I have done this with Ubuntu 10.04 (Gnome 2). Create a new user which will be used for this test. Make sure you have keyboard shortcuts for password protected scripts to launch gconf-editor, logout, and perhaps the keyboard shortcut editor. Disable all other keyboard shortcuts, remove menu bars, disable the desktop.
Now set the user's profile to launch the desired application upon startup. They can close the application at their own risk, or you can control this too.
Once the profile is satisfactory, replicate as needed.