There's plenty of evidence most people can't/don't want to do full self custody. So if "everyone" adopts them they will inevitably have the coins managed by third parties. Plus everyone including many people with self-custody have expectations about fair conflict resolution and fraud prevention. Even if they are in denial till stuff hits the fan. That's where and how governments step in.
As a technological matter, disregarding probable human behavior, can it be done?
Yes, and even easier than tradfi since the ledger is by definition public. At that point it's allbjust passing dicta from on high. The crypto nutters kind of played themselves if they thought their medium was taxation proof.
Maybe, except taxation is not a technological matter anyway.