If you own 80% of something worth $1B - you have $800M. If you can't sell $2M at your preferred price to pay your taxes, then it's not worth $1B, and you wouldn't have to pay that much tax anyway.
But the whole point is that you don't _want_ to sell it. And it's entirely possible to own something that is work more than 20x (given a 5% tax rate) than you currently have in accessible cash.
For example, assume you write a story and MegaCorp decides they want to make a movie of it. Right now, they need to pay you to license it. With the Harberger tax scheme, they can force you to sell it to them; because you don't currently have the money to pay taxes on what it's worth to them.
If someone just wants to create something they like, and then own what they created, a big corp or rich jerk could easily bankrupt them through the tax/force them to sell it off.
Some things don't need to exist in the context of a market and markets are often less efficient than folks seem to assume.
1. Private — untaxed, lasts for 14 years (or until the death of the last author), goes into Public Domain. No need for registration; and,
2. Public — any work (before it's expired) an be transferred to a Public work. At that point some sort of Harberger tax scheme would kick in.
Obviously, picking a number like "14" distorts the market — works that take off after 14 years are going to be undervalued, which sucks. OTOH, the point of copyright is that the opposite distortion — free-for-all — is considered bad.