I really don't see why "equity is volatile" is some insurmountable problem. We see the same exact "problem" show up with day trading, where it is possible to accumulate a lot of taxable gains and end up with less than you owe if you don't sock away the money to pay for taxes at the time of locking in the gains.
If an equity is valued at $X on some date when we lock in the value for a wealth tax or a tax on unrealized gains then sell enough of your equities on that date to pay for the tax and store that in a zero-risk asset. This can easily be done automatically.
If an equity is valued at $X on some date when we lock in the value for a wealth tax or a tax on unrealized gains then sell enough of your equities on that date to pay for the tax and store that in a zero-risk asset. This can easily be done automatically.