It's interesting (and anxiety-inducing) to imagine the forms this might take, given the different offense / defense tradeoffs that btc introduces. In the era you describe, it was a lot more clear who had financial assets to be seized, and it was more clear when you had seized them.
What happens when it's less clear? What do the Power That Be do to restrict capital flight? Unpleasant to think about but important to try to get in front of.
I think nothing can be done when you are under the power of someone else. Money, no matter fiat or bitcoin, is just a tool. To stay free you need power, not money.