Hi Charles,
The two biggest issues you will face making your own device will be back focus and rigidity, and I believe the exact mirror placement is quite critical too. I think the ONAGs mirror is laser aligned. The BF gets tricky to say the least if you also need to fit a reducer or flattener. Putting those optics before the mirror seems best in my view but then BF can be a game stopper. If you need to place the camera very close to the body of the ONAG like device there can be clearance issues. If you place after the mirror you might get more vignetting but you also then effect the guide camera focus (which also needs to be extended. When the imaging camera is close then so must also be the guide camera chip. The latter can be tricky. It's not insurmountable for sure but you need to calculate carefully in advance.
Another issue of high importance is how you intend to focus the guide camera? To use FocusLock the guide camera needs to be focused critically...not by hand which is not nearly exact enough. The IF helical focuser is good (and expensive) but so is Garry Jarrett's KISS focuser (much less expensive). Gary's focuser is very rigid and allows the guide camera to get very close (much closer than the IF helical model). Gary will probably work with you to make an adapter to mount his focuser to whatever device you end up modifying. I've used Gary's focuser with the ST-i and Lodestar, and it should operate perfectly with any camera having similar size barrel (1.25").
This is a recent setup though I have added a TAK flattener in front of the ONAG-XT. This forced me to swap guide cameras as I now need to get extremely close to the back of the XT model. Gary's focuser saved the day!
http://www.pbase.com/prejto/image/160513305
Let me know if I can assist you making this up!
Peter