Yes, i thought a bit more about the idea, (during the night hehe)
and i figured that the vignetting/light falloff is not ideal if the mask is not centered. However, on a 6x9, the lens is supposed to cover 9 cm in BOTH horizontal and vertical directions...that is a circle of 11 cm approx., so a 3x9 cm panoramic with the top or bottom of the gate covered, should not show major differences in light falloff in its top vs bottom corners, unless the light falloff is seriously bad on a 6x9 frame too. In panoramic, i'd use small apertures too, for a large DOF, and that helps for less falloff/vignetting as well.
Anyway, i'm curious about the results of Dingo's modified camera, and maybe i'll try to do the thing with the 6x9 bessa i'm planning to get soon. Sorry to steal your thread, Dingo🙂