It's not like the money is just sitting, liquid in a vault like Scrooge McDuck.
Idea: tax loans taken out using assets as collateral at regular income tax rates. After all, that money gets used like regular income (living expenses).
The taxed amount can then be added to the basis when the asset is sold. It would be like reverse of depreciation calculations.
Set an asset and loan value floor so it only affects people with assets $10M+.
After all, regular people pay taxes on annuities, which are similar in structure.
Disclaimer: IANA-Accountant, but I am a taxpayer who tries to legally minimize my taxes.