Obviously, I do not know nitty gritty details of economy and finance, but if I would implement this tomorrow I would start taking the equivalent of surplus and start from there to understand more.

For example, say individual has 20M liquid cash, 2 houses each valued at 5M and 1.5B in company shares (based on averaged company value for the last 6 or 12 months):

* whatever you can immediately spend is prioritised first, so you keep your 20M + 2 houses, then surplus is $530M of your company shares

* this equivalent number of shares will be moved to government trust, individual doesn't have any control over it, if person dies next day, government keeps the money (lets simplify for now and keep voting rights as separate question)

* let's say after shares moved to gov. trust, during next 6 months company value halved, gov. returns all your shares, if stock dropped only 10%, you get equivalent back to make your net worth 1B

* regarding taxation, I would keep it as it is today and tax on "realization event"

There are around 3.000 billionaires in the world, even hiring 10 dedicated people for each billionaire to calculate all this stuff on a quarterly basis is not expensive