Just stumbled on this thread. One aspect of your question about reducing the frame to a 645 doesn't seem like it was addressed. It is true that the location of the window will result in the same amount of film being wound on as when you are shooting 6X6.
If, IF, you make a portrait mask (ie with the long axis verical) you will have greater space between smaller frames. But, you can also choose to make the make landscape (ie with the long axis horizontal). This would give you a very simple and convenient system for shooting landscape orientations without having to rotate the camers as is necessary with many modern 645.
It is something I have wanted to try, because I like the aspect ratio better. If you do make a frame, please let us know what material you used, how you fixed it in the film gate,and what your results were like.
Oh, yes, and you can mask the finder down to reflect the frame. Mask the top and bottom 13% (tape on the front of the viewfinder should do it) , or make it 10% if you can live with the imprecision.
Giorgio