The ump checking the bat does not absolve the bad owner or the company/person that rolled the bat from liability. It is absurd to believe that the ump can or should have the ability to "proof" every bat. If you want to claim that is the best process, be prepared to pay a couple hundred bucks per bat per game for a portable lab to inspect/xray/weigh/test each bat. Absurd? Yes, but that is what it would take if you want to have field inspections that assume liability. And, yes, per game depending on the bat quarantine procedures. Hey, sounds like a heck of a business opportunity !!
Reality is that the person playing with the bat, the person who paid for having the bat modified and the person doing the modification should be made liable. Passing off the responsibility to the ump is not a viable tact.