Why would this matter? The extra can be easily cropped in printing without having to monkey with the internals of your camera, and that way you could fudge a little this way or a little that way if desired. The masked viewfinder would simply provide a nice and helpful compositional aid, but masking the negative itself is totally unnecessary. That is like sanding the grooves flat on side B of a 45 because you only want to listen to side A.
Yes, and, while I'd never think of sanding down a 45, I was replying to the original poster's inquiry, which wasn't, "How can I view the world in a certain aspect ratio, " but,
"Is there any way to get such an aspect ratio, without cropping, on still 35mm?"
Myself, sure, if I wanted a certain ratio, I'd probably just crop.