Hold on a tick, I was under the impression that Bola balls are as soft if not softer than a regular cricket ball. In which case why is it such a problem? Wouldn't it be like using an old ball which is recommended anyway? And instead of looking for seam marks, you're looking for dimples.
And if Bola balls are harder than a cricket ball, doesn't it make it more like using a mallet? Which people use to knock in bats?
So not sure what the issue is, regardless of the hardness of a Bola ball. I myself haven't used a bowling machine purely to knock in or open up bats, but I do like the idea.