It's fantastic to see these older threads get up on their legs and run again. The Big Wheel keeps turning. The newest photographers get acquainted with the vocabulary and culture and materials of the older photographers, all with an eye on the potential for the latest technology to keep the older technologies alive -- at least as artisan niches. And, besides, it gives us something to talk about!

I'll throw in a couple of thoughts. 1) I don't think backing paper is an issue. Thin, acid-free, black paper is available from the Paper Art world.

2) When you're designing a sprocket hole puncher, keep in mind that it will probably work best to first coat and dry the film and then slice and punch it. Otherwise, you'll fill the sprocket holes with emulsion, which is nearly as hard as PET. (I'm assuming small scale machinery all along the process.) That means the hole punching will have to be done in the dark, and care taken to prevent scratches on the emulsion.