To create a custom display, I want toplace widgets in arbitrary locations, or move them around rapidly, or draw to them directly. How?

You are probably fighting a losing battle. Widgets really aren't what you want, most likely. Consider using a GtkDrawingArea or the GnomeCanvas to create your custom display.

If you really need interactive widgets, such as a GtkEntry or GtkButton, you can try to use GtkLayout or GtkFixed.

If you have very specialized needs, you probably need to write your own widget. the chapter called Writing a GtkWidget tells you how to do so.