Well, how would that work.

It probably wouldn't. That's my point. We independently value real estate for property tax purposes, but that requires a fair amount of effort even for a fairly easy asset class to value (it's immobile, open and visible, all transfers have to be reported timely to the government, there are frequently comps and related sales, and as a single asset class it's easy to assemble expertise). It would be very difficult to replicate that to create independent valuations for things like privately held companies, artworks, gemstones, mineral extraction leaseholds, and a host of privately held intangible and intellectual properties (like when Michael Jackson bought most of the Beatles catalog for about $48 million - what's that worth today?). Much more difficult to update them over time.

In that respect, Buffett's the simplest case. Almost all his wealth is in publicly-traded securities, which are easy to assign a market value to. Much harder to put a value on, say, the unrealized gain of Jerry Jones for his ownership share of the Dallas Cowboys or George R.R. Martin's ownership of the copyright and licensing rights to his Game of Thrones characters.