Most capital owned by billionaires is not taxed until it is sold, so in the case of Hashimoto and others they most likely have not paid tax on the majority of their wealth.
> How would the "very small wealth tax" be calculated, that you propose?
In the same way we calculate income tax, we make it up. Most numbers I see are between 1-3%. We could just start at 1% as that is the most conservative number.
It is possible to tax unrealized assets. We already do. For example, a property owner pays property tax based on the value of their property, even when they are not selling it.
It is possible for billionaires to borrow against their held assets. It is therefore also possible to calculate a tax on them.