Yay! I've just committed the GtkMenuToolButton widget to GTK+ HEAD. Thanks to the help of federico and maclas I even managed to defeat the documentation build system vodoo and commit the basic docs for the widget... the widget should be straightforward to use anyway.
Having the widget in HEAD means that it will be in gtk 2.6 and thus it is available for Gnome 2.10 (or should I say 2.A?).
Now I only need to make some various projects use it.