First, even many of the refractors that are "Binoviewer Ready" will still require you to use a T2 diagonal with the binoviewer connected directly to the top of the diagonal (either a T2 ready or T2 modified).
Second, my advice would be to post a question like "show me your binoviwer ready refractor with the binoviewers on it.) The reason I recommend that you do this is because the Binoviewer forum does not get enough traffic to get a speedy, in-depth answer to your question.
As to your question, here are the two refractors that I can tell you that I used that I was able to get to work with a binoviewer.
The ES 127ED reached focus with one of the focuser tube extender removed. I was using the Baader standard Prism diagonal along with a Mark V and I only had a very few millimeters left, so this means that no all binoviewers would have reached focus. For example, the Televue Bino Vue is T2 ready, but it has a light path that is around 130mm and I am not positive that it would have reached focus. I only had about 5mm left with the Mark V, and for the size prism, the Mark V has a tidy light path.
The next scope was the Stellarvue 110. This BV easily reached focus with any T2 prism and any T2 binoviewer I put on it. The SV110 comes with a 110 extension (as I recall) on it, and I took that off and tried to do it native, but that made for two problems and there is on important note on this configuration. For this to work, I had to use a 2" diagonal. There was not enough out-focus for the 1.25" diagonal (This could be solved with some kind of extension tube on the T2 diagonal, but I did not see the point of that. The next issue was that the focuser tube was so far into the light cone that the field illumination falloff was quite high Now the 2" diagonal is good news in that almost everyone has a 2" diagonal, so that means nothing else to buy, but again, my issue was that the field illumination was pretty high (vignetting). Not everyone is bothered by field illumination falloff though. If you are under very dark skies, you probably would not notice it so much, but under bright skies, you are viewing a bright hole through a dark doughnut.
The configuration I eventually used was to order a 55mm extension from Stellarvue (I think it was 55mm, not positive). With this on, I was able to reach focus with any T2 setup and as I recall, I could even remove the BV and go to a 2" diagonal and still reach focus.
Now I have converted two refractors to be binoviewer ready and this is quite inexpensive. to do. Both were 4" achromats and I simply cut about 2" off of the tube of each one. I sold both of them and since they were inexpensive achromats, the suffered no penalty for the modification because in the end, both were bought by people that wanted a binoviewer ready 4" refractor.
Not all larger telescopes are refractor ready. If the scope was designed for imaging, then there is a good chance it will work. Many of these scopes have extra in-travel to accommodate focal reducers and many have oversize focuser tubes to provide good illumination to large sensors.
I would be wary about the advice that almost all larger refractors will reach focus with binoviewer though because none of the larger telescopes I have ever owned would do so without using some kind of GPC.
Again, I would recommend that you post this on the refractor forum. I know this seems like the right place for it, but this forum simply does not get enough traffic for you to hear about all of the candidates and hear about what configuration was being used.
Edited by Eddgie, 02 October 2019 - 08:09 AM.