It is now possible to invoke NSInputText methods from automated tests. It is also necessary to test how NSHTMLView calls NSInputManager, so a custom NSInputManager is needed in DumpRenderTree. It could be as simple as a logger, similar to editing delegates - or, better, provide a way to use arbitrary JavaScript functions as NSInputManager methods.
Oliver's patch in bug 14710 takes a somewhat different approach, but it also lets one write custom JS input methods for testing.