Mark, I don't know what it was, however, I've got it working now. The Exception thrown in my version of FormAuthenticator is thrown now YESSS!!!
My best guess is that I mixed up some directories when building the jar containing my Authenticator-classes, anyways, subclassing FormAuthenticator and patching Authenticator.properties works for me now :) Many, many thanks again, and all the best for your Struts-prob! Greg -- what's puzzlin' you, is the nature of my game