That would be a very flexible approach; 6*9 is a useful size, same aspect ratio as 135, but umpteen times the size. I'm guessing it would be unwise to change in the middle of a roll. Or at least, care winding on the previous frame would be needed.
I did wonder about red window winding when I started thinking about what a 6*12 back would look like. Trouble is, I really hate red window winding (i's why my late Dad's Ikonta doesn't get the use it deserves), and it would be quite hard to use in blue hour situations... although Andy's little red torch might work, I suppose!