After several weeks of work, I think I have a working implementation (very basic so far, and lots of caveats..) of something that solves this problem. To recap:

you want to prove that you own a total of T BTC, but privately. But revealing exact amount may make it too easy to deanon. So I have a proof of this statement: "I own N utxos, whose total value is between k and k + 2^n, for some k and n that I announce, but I am not revealing which N utxos they are, or what their individual values is, out of a given list of 300k utxos". The code at https://github.com/AdamISZ/aut-ct/tree/auditing now does this.

See the latest commit note. In brief you need 3 or 4 components to the proof: 1/ a proof that each blinded commitment is part of that 300k set, 2/a proof that each committed value, when summed together, lies in the given range (using bulletproofs), and 3/ key images per utxo to ensure you dont' cheat by just claiming the same utxo multiple times.

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