John, I didn't ask about modifying a back for pano work. But I'm not sure your suggestion will work. The roll holders I'm familiar with don't have the real estate needed to enlarge the gate. Bryce Alvarez, who's told me what he went through to make a long holder out of a couple of Adapt-A-Roll 620s, made it seem very difficult. He's a good machinist, said he wouldn't do it again.
Originally Posted by jnanian
I'm with you on the DaYi, except that I can't convince myself to add a 4x5 Crown to the kit.
Another possibility if the goal is simply a long low image is to convert a Folding Pocket Kodak to 120 film. Its been done, but the result is effectively a crop from a shot taken with a normal lens.