It would depend on the size and shape of the ball bag, and the balloon. You might not get full inflation or full pressure against the leather using just any balloon.
My first bag was made from the hide from an old softball. My second is made from Elk Hide left overs I have from making pants. I made the Elk Bag Too Round, and Too Big. It would have been a great candidate for using a balloon, but it holds too much shot, and is cumbersome to hold in use. I have a ball bag made from rawhide that we used sand to form. It also is too big.
I like the size of the bag show above. But I also like the ball bags that are narrow in shape. They fit better in your hand, you don't need to carry so many balls or as much shot, on most hunts, and they don't make you lean to one side when you carry them in a pocket or over your shoulder walking into the field.
I would rather have several shot bags than just one. That way, I could carry what I need into a field, and leave the rest in my car, or back in camp. I could " re-supply" if needed.
Only dove hunting requires a lot of shot in the field, because of the large daily bag limits. But, I carry a bucket to sit on, and it can carry extra shot bags for that purpose.