[eluser]tonanbarbarian[/eluser]
the issue is that in a lot of cases (most actually) the controller is not loaded when the 404 error is processed
this is because CI processes the route to determine the controller to load. if the router is unable to determine a controller it goes to 404
however this is nothing stopping you from loading the helper in other ways
you can determine the path to the helper yourself and include it
looking in the system/libraries/Loader.php for the helper method will give you some ideas as to the paths you need to check and or include to load a helper