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 menu button also shows up for Chrome and Firefox.
The about to show hack seems to still be neccessary for Firefox and Chrome - as soon as opening a sub menu it closes in both of them. Needs further investigation, probably reintroducing the aforementioned hack. :/