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.