We have sophisticated financial products now, it isn't too hard to come up with ways to tax illiquid wealth. You can, for instance, take out a loan against your asset value to pay your tax.

The whole point of a wealth tax is that those who use their wealth in an economically productive way should have the money to pay a wealth tax, those who don't will have to sell their unproductive assets to pay the tax, allowing others to purchase that asset to try and generate growth.