I've come a cross another bug with the way the WLC compiles the webauth bundle. Version 18.104.22.168. If I make any changes to the generic login error codes I get a compile error "Error extracting webauth files.".
At this point I'll avoid airing my grievances over the buggy nature and lack of development for customised webauth bundles. And simply say that I'm not even looking for a workaround now.. screw it. You're stuck with the naff green login screen and naff error codes mr customer.
And no... recommending ISE will not help me so just don't do it