Putting a mask between the film and focal plane shutter will be the very difficult, of not impossible task.
I'm not offering solutions - sorry for that - but do think that cropping later is the much better way.
You can still aid composition by adding masks on the focussing screen without much trouble.
You only have to match the crop to what you saw in the viewfinder in the darkroom (cropping is not a digital technique, so no need to give up your LOVE for analog
). A bit annoying perhaps, but it is there on film.
Besides: it gives you a second chance to rethink the composition, maybe find there are more than just one in the whole negative.