This strikes me as something that should be in the manual. Would everyone
agree it would be worth lodging a bug report to have this information added as
a footnote or reference i.e. that you cannot add menu actions/buttons to the
top level menu when using gtk_?
Yes, sounds like useful info to me. And info that it might
not be easy to discover otherwise (modulo writing to emacs-devel as you did).
Even if this is not Emacs-specific but is general to GTK, an Emacs user thinks
of the menus as Emacs menus, not necessarily as GTK
menus.