Why do you consider internal focus an advantage for astro? The ones I have used are slower than a good rack and pinion with a two speed.
I consider it an advantage in terms of comfort, although It depends on your observation habits and material... In my case I observe with my glasses resting on the eye-piece and when I zoom my bino combo http://www.pt-ducks....#CR-binoviewing
I have to move my head - those that use Denk power-switch or Siebert PMW, know what I mean - again in my case, from low to high zoom limits with the bino combo I need almost 50mm focus travel.
It would have to have very large prisms to avoid vignetting with an eyepiece like the 31 mm Nagler. Balance and mounting are issues too when using those big, well corrected eye pieces.
If you had followed my previous Nikon FSA-L2 link you would noticed that it works with the UWA 2" 30mm without vignetting. By the way, it uses lenses instead of prisms - results longer but allows >3x zoom factor
After your comment I remembered to add the Celestron Axion 31 contact info on the http://www.pt-ducks....htm#Test of 82º
,%20100%C2%BA%20and%20102%C2%BA%20AFOV%20zooms - note that the ES82 30mm didn't work well with my combo.
You are right when mentioning that balance and mounting issues appear with these large eps but is doable although I would prefer a much shorter combo...