It’s all a giant scattered portfolio, funded largely by loans against stocks which make up most of their compensation.

Yep. A solution is a tax on registered securities. Any individual holding more than $100 million in registered securities owes 5% on that excess value, per year.

Most of the portfolio cannot be easily liquidated, at least not in large amounts, without serious rippling effects in the economy.

Nah. They don't have to liquidate their holdings. We'll go ahead and do that for them. We'll take 5% of each of their positions each year. Transfer them to an IRS liquidator, who will sell them off over months, years, decades if needed, in small quantities, no more than 5% of traded volume per month.