I've started a very basic developer guide, based on parts of Brian Foster's Workflow UserGuide. Let me know what you think.
Wanted to mention the following though:
 I've left 'Architecture' section blank for now. I don't think the UserGuide really explained the architecture of CAS-PGE in detail, or perhaps I just did not recognize it in the text.
 The image (pge_instance_plugin_points.png) image attached is in a different diagram style than most OODT documentation diagrams. If you could let me know which tool was used to generate images in documentation like Workflow components, I'd be happy to convert the attached image to the desired style.
 We would need an example for the user to follow. I think that should go in a separate page called 'Basic Users Guide'.