Me and a friend of mine made a prototype for one, using the internals of an impossible project phone ‘printer’. It takes standard 600 film. I had no idea that it would be the kind of thing hacker news would be interested in though.
Search the youtube and you will find a bunch of hacks people have done to use Instax film in MF and 4x5 cameras. I recall hearing some time ago of an attempt to make a Graflex back for Instax as a commercial product, not sure if that actually happened.