Features in KDE Base
first success; catching at least existing menus!
Menus from programs started later still are mysteriously not seen.
The embedding is done asynchronously, I found out.
So just kick the embedding in to start and just behave, as if the menu will be there in short time.
If not, just catch the fail signal, which could also happen later on and needs treatment (or is closedClient emitted then, too?).