We all know that. But there has been discussion of making an Instax back for 4x5 cameras, at least, maybe others. Apparently they have been done as one-offs and at least one person or small company is working on one as a potential commercial product. So if we can't shoot peel apart maybe we will be able to shoot Instax in our "regular" cameras before long.
I was just saying I think the stuff is too darned small to be very useful on a 4x5 camera, and the number of medium format cameras on the market might limit the practicality of making backs for those. Or maybe not - limit it to 6x7 and 6x6 (maybe the Fuji 6x8s) cameras with interchangeable backs and what does that leave? RB/RZ 67, Hasseleblad, Bronica SQ and what's the Bronica 6x7? I think those would cover the majority of medium format uses. It would actually be fine too on my 645 - I use the peel apart some now for proofing and the image, though it wastes a lot of the quarter plate sized FP-100c, is actually about the same size as Instax anyway. But that opens up more cameras needing custom backs, at minimum the Mamiya 645 Pro and TL and the Bronica ETR series.