I just discovered the appmenu feature in Debian's KDE (it comes with appmenu-qt package).

1. Is there some standard keyboard shortcut for showing (toggling?) the appmenu?

2. While the idea itself is interesting, it's not always comfortable to use. It works OK with fullscreen windows, but with small windows closer to the bottom of the screen it becomes uncomfortable. I'd prefer an option for the menu to appear not on top of the screen, but in the window title bar for example. Is it possible or planned? (Note: it's not the same as menu from the button in the title bar, what I want is something similar to the hide menubar Firefox add-on). If it's not planned, I can file a bug about it.

re #2: see viewtopic.php?f=83&t=117385&p=295709&hilit=appmenu#p295709

And of course you can already configure it as a button in the window's title bar.
Just change the setting to "Title bar button" instead of "Top screen menu bar".

Yes, but I already pointed out above, that button in the title bar is not the same as toggleable horizontal menu in / under the title bar. Any idea about the keyboard shortcuts by the way?