How about having your doHelp() method call your doView() method with an extra attribute, then have the doView() pop a help window or overlay? Or forget the help mode and just embed a help icon in your view mode.
Thanks for the suggestion. We had already tried this and it !sort of! works, but not quite. It does pop the help window and the portlet content remains intact but it also replaces the help icon in the title bar with a "CANCEL" icon (and, incidentally, scrambles the title bar a bit, which I think is another bug altogether because we've seen it when we use the normal JBoss portlet help mechanism, no subterfuge required.) The solution we've implemented is to create a "help" portlet and install instances of it on each of the pages where we need more comprehensive explanation. When a user clicks the help button the help window appears (via a processAction call) and no other portlets on the page are disturbed. The help portlet determines the page where its button was clicked and generates the right help content URL.