Hi Bill,
Check your calculations carefully! My ONAG -XT BF is 69mm and I'm certain IF did everything possible to reduce BF to the absolute minimum. 90 mm means that you must consider 2 adapters (one for each end of your "ONAG") + camera BF with filter wheel. Even the ATK ONE requires 27mm BF with inbuilt FW. On my TEC180 the best location for the TAK TOA-67 was ca 112mm. from the CCD. My camera is as flush as possible on the ONAG and I had to custom design a special fitting (Precise Parts couldn't do it!) to mount the TOA-67 (saved me ca 4 mm). Bottom line, I'm not sure how you will get an ONAG like device with smaller BF than around 68-70mm unless you use a smaller mirror. You might get away with that if your imaging camera CCD is small.
This photo shows my current setup using the TAK flattener. Notice how close the imaging camera is to the ONAG body and how close the TOA-67 is. It just fit when I shaved off 1.5mm from the Moravian t-thread adapter.
http://www.pbase.com/prejto/image/162166083
The other question is on your newt scope do you actually have enough in focus to mount ca 100 mm of gear? You might need to make too many modifications to your secondary. The ONAG seems to work best on the CAT type scopes.
Peter