They could open their books to an auditor and allow the auditor to render an opinion without disclosing any confidential data. This is how the rest of the world does it.
To finish the thought if not clear: If MtGox goes belly up, the auditor can then be sued for damages, and put up a bond. Of course, the auditor is going to require a fee to do this.
Pick your poison: do you want to pay higher fees for a little more security, or place a discount on MtGox funds that you lay claim to.