Ingredients:
1 packfilm camera, empty (me: a Polaroid Automatic 100)
1 integral film camera, empty (me: an SX70)
1 empty packfilm case (me: FP100C)
1 empty integral film case WITH a working battery (me: IP)
integral film of choice (me: IP black & magenta 600)
darkroom/changing bag
Method:
1. Disassemble the packfilm case. I don't know about other packfilm cases, but the Fuji stuff comes apart really easily into 3 pieces.
2. In a dark room/bag, remove the darkslide from your integral film and pull out 1 exposure. At this point, I replace the darkslide so that I can pull out the packfilm for later use.
3. Load the integral film into the packfilm case. It's not an exact fit, and your integral film will bow out a bit, but it should look something like this:
Definitely practice with an exposed shot in the light first
4. Still in the dark room/bag, load the assembled pack into your packfilm camera.
5. Now you can take everything out of the dark and shoot your single exposure.
6. Bring the packfilm camera, integral camera, and empty integral film case back into the dark, remove the packfilm, disassemble the pack, load the film into the integral case, and load it into the integral camera. The camera should automatically spit out the film, thinking it's the darkslide.
7. You're done! Bonus: no need for a frogtongue because you're in the dark already!
Obviously, the picture will be L-R reversed, but you can avoid the up-down reversal by orienting the camera so that the bottom of the film is facing up (something I forgot to do in the first test shot below).
Another advantage is that you can do multiple exposures, like I did on the second shot. Exposures are off because my camera doesn't have a 600 setting so I set it on 300 and turned the exposure compensation to 100% dark.
As a proof of concept it works! Definitely a lot of effort for one exposure, but if I can figure out how to make a darkslide for the packfilm, then I can load up a bunch of packs. Getting the darkslide back onto the exposed shot, however, will still require a changing bag...