It was quite hard to figure out how to access a menu's action for a menu that has been defined in a ...ui.rc file. I didn't get any answer on the KDE forums, but luckily from the kde-devel mailing list after all. Imo, this is almost impossible to figure out, at least if you're not completely into the KDE internals.
I thus added a small note on how to do this to the API doc for KActionCollection::action, as this was the first place I searched for how to do it.
Maybe, you want to add this :-)