Proof of XTZ Holdings

FTX did not hold the Bitcoin they should have. Some exchanges do not vote with their XTZ coins as they do not wish to participate in governance. Perhaps Tezos would benefit from a Proof of Holding vote, where the vote is only used to prove how many coins an exchange owns. I have my doubts about a couple of them and I would feel better if they proved it on-chain. Some of bakeries do not seem to have the quantity I would expect with their exchange volume. There are some possible reasons for this, but I would feel better if exchanges proved it.