I wonder whether perhaps the roller plate holder could be used for this purpose. I recall someone did this previously with instax (wide?)
You can use current packfilm and maybe cutfilm holders, but it's not an easy method. In the dark, or inside a dark bag, you load the instax cartridge into the holder, then put darkslide, then mount to your MF camera, and take your picture. Then in the dark or dark bag you take the instax cartridge out of the holder, then pop it into an instax camera. You want an instax camera that automatically ejects the darkslide (or first frame) that way it ejects the picture you took. This is too cumbersome for me..
Until I save for a 3d printer, I'm currently trying to source a broken/spare polaroid back for the Mamiya Universal. It has a removable adapter plate that I can remove and graft onto some kind of instax back. But this will be for Instax Wide, preferably. If I can make that work then I will try it with square. The hard part is getting a donor back. For Instax Wide I will need to chop up a 210 or Lomo'Instant Wide ($$$).. the Lomo probably the better candidate since you can get a flush mounting surface at the film plane. For the square format, are there any cheapy cameras yet/coming or is there only going to be the SQ10?