If you must use one of these widgets that are undocumented, I strongly suggest you take a look at the gtk.py files in the PyGTK distribution. PyGTK's method names are very descriptive. Once you have an understanding of how things work, it's not difficult to figure out how to use a widget simply by looking at its method definitions. This, along with a few examples from others' code, and it should be no problem.
When you do come to understand all the methods of a new undocumented widget, please consider writing a tutorial on it so others may benefit from your time.