Proof of reserves for a DTC-traded stock like MSTR would be really complicated because there's already the issue of proving what percentage of shares each shareholder owns, in addition to proving how much Bitcoin each share owns.
Proving 1 share = 1 share is fairly simple for a commodity ETF because the price is supposed to closely track the value of the ETF's holdings of the commodity, so you'd know if your percentage of the ETF is diluted by the DTC when you notice the ETF bleeding badly compared to its underlying commodity's market value.
MSTR is not an ETF and if it bleeds badly compared to Bitcoin's market value, the average investor just goes on wallstreetbets to bitch about how they don't understand the price action.