Well, that approach will not do what you want. However, with a small filter addition to the cookie autorization it is possible to make a plugin that will deny the logon cookie for the front end.
Such a plugin is now available in tonight's build. You may not really like it, though. It makes it so that an admin is actually not logged in when viewing the front end.
BTW, it would be possible to extend the plugin to list which admins had this option. Not sure if that is actually useful, though.