"allocated storage" for online wallets and exchanges

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It's probably been discussed before, but why isn't anyone doing something for bitcoin analogous to the allocated storage common for gold?

I found a discussion on reddit about it, and everyone was talking about hash trees, wallet services having to disclose all their holdings, and other issues, but aren't they over-thinking it? Am I misunderstanding bitcoin, or would it be as simple as the customer providing a unique identifier, and the storage service transferring the amount the user requests allocated, including the identifier in the transaction? Then any third party could verify the transaction identifier, and monitor for spending from the allocated address.