Not sure this is easily doable in GTK2. However, I learned recently that there's the GtkLayout widget which allows pixel exact rendering of widgets, so you can even display some widgets over others. Or you may implement your own container widget.
Please note that since GTK3, there's GtkOverlay which seem to do what you want.
You may also give a look to Clutter, which might allow this. Furthermore, there are projects of merging GTK and Clutter for GTK 4.