I am the same, I saw a couple of 70mm backs in the past which I let pass by. Hopefully a Bronica one surfaces up at some point.
This said having 70mm format available would be a god send for re-spooling for vintage cameras and odd formats. I had looked for 70mm in the past without success, so I'd be frilled to get hold of some.
I am still investigating how I would process it. I sent an email to Hewes to get the dimensions of their reel. I wonder whether I could use a jobo print tank to that effect.