I know that the API tells us that #setUseSystemMenuBar() is meant to render the menu popups as real system internal menus. (Ok: On Linux this flag has no impact at all so it seems that this platform does not support this yet.)

Ignoring the API documentation I would assume, that the complete menu is rendered by the system - for example as the global menu of Mac or Ubuntu Unity.
I have to admid, that I would like this idea since for me there no reason to use the flag at all if only the popup is rendered by the system.

Thanks for taking this into serious consideration - even if you decide that this is not worth the work :-)