(this is based on a suggestion by jrb and andersca)

libglade should be able to work with things other than gtk widgets.  With
the development version, most of the code is simply working with GObject
APIs rather than GtkWidget specific APIs.

Being able to handle things other than GtkWidgets should be quite easy,
although it will most likely involve API additions (which is why it is
probably too late for 2.0).  Here is the basic steps:

  1) remove assertions and assumptions that
     glade_xml_standard_build_widget(), etc are working with GtkWidgets,
     where appropriate, so that they can work with arbitrary GObjects.

  2) Maybe rename functions with widget in their name to indicate that they
     don't just work with widgets.  This would be acompanied with
     compatibility defines, so that programs compiled with the new lib
     use the new functions, and with compatibility functions in the lib
     so that old programs continue to work.

  3) add an <object> tag to the glade file format, that would be treated
     exactly like <widget>.  This would mainly be cosmetic, but it feels
     a bit weird defining non widgets with the <widget> tag.   Handling
     of the new tag would be identical.

Possible uses for this would include:

  - GtkSizeGroups:
      <object class="GtkSizeGroup" id="sizegroup1">
        ...
      </object>
      ...
      <widget id="blah">
        <property name="size-group">sizegroup1</property>
        ...
      </widget>

  - Specifying GnomeCanvasItem objects in a GnomeCanvas widget
      <widget class="GnomeCanvas" id="canvas1">
        <child internal-child="canvas-root">
          <object class="GnomeCanvasGroup" id="group1">
            <child>
              <object class="GnomeCanvasRect" id="rect1">
       ...

  - GtkTreeViewColumn objects in a GtkTreeView widget
  - GtkCellRenderer objects in a GtkTreeViewColumn object

Probably other uses that I haven't thought of yet.

This is almost definitely not 2.0 material though.