Seems like a great idea, two question spring to mind:
1. Are all typical guide cameras sensitive to the pass through wavelengths your gear provides? For example I use a Meade DSI II Pro mono camera - would that work fine to guide with your gear?
2. How does one achieve fine focus on the guide camera? Is it a lot easier as you're right on the main light path - so will using a Bhatinov mask give a better illuminated image on axis rather than off axis - and how do you fine tune your ONAG camera's position once your main camera is correctly focused?
Smart idea!
Manual is very helpful:
http://www.innovationsforesight.com/...anual_V2_7.pdf