How can the provable solvency scheme possible prove solvency for fiat holdings? For example, I deposit $100 into the exchange. How can the exchange prove to me that it hasn't gone and gambled that $100? I think this could only be handled with human auditors going in and checking the actual amounts held in the exchange's bank accounts.
Can't, except as you note. Best trade your fiat for something actually auditable through cryptographic means ASAP. :)
(for a less quippy answer: they could show their fiat liabilities through these means and have a third party bank attest to their fiat account assets (or an encryption of their fiat assets). At least then you'd be trusting some established bank and not just some sketchy cryptocurrency industry company)
