I guess it would make sense to agree to list all player balances greater than a certain amount (like 0.1% of the bank?) each time you do your proof of solvency.
I'd be all for regular proof-of-solvency of uninvested player and investor balances. Only showing investor balances does provide opportunities for some sites to practice fractional reserves if uninvested balanced grows to be a larger part of the balance. With this being not public information in standard practice.