I agree this would certainly be better than an outright ban on price gouging, though I don’t understand the advantage over not treating disasters differently.
The larger the tax base, ceteris peribus, the less distortionary the tax, so if you want more money for the poor during disasters, just raise normal sales/income/or property tax rate an amount to equal the revenue needed to help poorer people in disasters, and then don’t have the more heavily distortionary high tax that only applies to disasters. As an added plus, doing it this way avoids a high tax when adherance is lower and may be more popular than an onerous tax when goods are at their most expensive.
I think it is much less bogus than you think. Yes, price would change if a large number sell or buy, perhaps that would be more of a statement on liquidity than bogusness.
At anytime, all shareholders are able to sell their shares at the market price, therefore anyone still owning believes the price is at or less than it’s true value, likewise anyone has the option to buy at the current price so (barring non-idealities) everyone else thinks the price is potentially higher than its true value, therefore market capitalization is much less bogus as it is the price where there is balance between people’s capital put towards buying and selling bets on the true value (of course lots of assumptions: low transaction costs, ignoring tax treatment, spherical cows, etc, etc).