Make a anonymized user list (I'll explain this below), publically post this list of users and their btc account balance, then sign messages to prove ownership of cold wallets.  The total amount of coins in your wallets should match the total amount of coins on the list.  Everybody can see their anonymized name in the list, and thus feels confident that he/she is included, and that the company has the funds.

Now the anonymization part: let users chose a password for the public balance, hash that password (or perhaps the concatenation of the username and that password to ensure that a certain combination doesn't occur twice), and use that hash as the username.  You could also give users the option to split their account into several anonymized usernames, if they don't like it that their total btc number is shown publically.