How about instead of taxing them any differently than now, we prevent them from borrowing against their assets? Force them to sell their assets and pay capital gains.
I think this would break all kinds of things, for example home equity loans.
The book about rules and regulations on this law would be larger than divorce law book.
Or better than that, loans backed by assets above some "jumbo" threshold ($10M?) triggers a capital gain on those assets in the collateral.
So if you get a $150M loan off of your amazon shares that on paper are worth $150M, but you paid $100M, you have a cap-gain of $40M, and at 20% tax, $8M fills the IRS's coffers.