Added the following to ~/.config/kdeglobals
[Appmenu Style]
Style=ButtonVertical
Applications like Kwrite, Dolphin, as well as VLC start exporting their manus on DBus now. The "showMenu" method on the org.kde.kappmenu dbus interface can be used to show the application menu.
The about to show hack seemed to have been neccessary for the global menu which is updated as soon as the active menu changes; right now calling showMenu does work with Firefox.