^^^
Cool idea. Does it have to be pneumatic? Sounds like a hydraulic system would work faster and more off the shelf. Possibly lower pressures too.
Same lateral expanding bladder, non expanding longitudinal ends...use an automotive or motorcycle master cylinder was your pump. DIY hydroforming.
It doesn't have to pneumatic. I think the key to success is the "balloon".
I'd still want that clamp/vice over the OD of the tube. The balloon is just suppose to help. I wouldn't want to over inflate and bulge the tube outward, either.