How would you keep Bitcoins 'in' a country in any meaningful way?

Well, you can't technically, but if Liberland is your only citizenship/only place your corperation exist no other country can tax your Bitcoins.

Provided that you don’t work/live in any other country and your business partners in other countries don’t have to pay taxes there, yes.

Cold storage wallets:

https://en.bitcoin.it/wiki/Cold_storage