I know there is no beam splitter. There is a point where the image beam either goes up to the viewer or back to the film plane. That's the split point. They are tied by geometry. Change the distance to the lens for one, you need to change the other. By adding distance to the back to put an adapter on, it changes the length to the lens. The distance to the viewing screen does not change. What is in focus on the view screen, is not in focus on the film plane.
I'm tired of trying explain it. When your pictures come out fuzzy, you know why.
If some guy has built an adapter for the 3x4 that does not change the length from the film plane to the lens, go for it. Me? I'll just use 3x4 film or my 120 roll film adapter.