I wrote already that my (layman) idea such a thing could be implemented is like checkpoints were handled. If a miner finds a block containing a transaction from or to a sanctioned address, it will then be simply invalid by the standard Core client protocol.
People can of course use the original protocol but then they will trigger the fork from this block on.
It's still an unlikely scenario but not completely impossible. I think if countries implement BTC as reserve on a large scale then the probability could be as high as 5%.
The problem is that states holding millions could scare much more with such a threat than ETFs holding the same amount, because ETFs always have to take into account their customers, which could run to a competitor if they believe an ETF investment company behaves in a wrong way.
States instead could say: "ok, the FATF is forcing us to sell our Bitcoins on the non-compliant chain, because otherwise we will be added to the gray list", and dump everything very fast.
I said already that I don't believe that ("the real") Bitcoin would die in such an event, but it could lose a lot of trust and value, and in an extreme case this could be used e.g. for 51% attacks, amplifying volatility and pushing back Bitcoin potentially by decades.
@stompix: Ok, I was guiding myself by the population and not GDP (and with that in mind the current $5-6k holdings would correspond more to something like 300k BTC for the US). But of course GDP is also a valid indicator for such a calculation. And even by population we would come into regions like 6 million BTC if every state followed El Salvador's strategy. You then may be right, El Salvador's holdings are already quite large.