There is already a loophole for players to switch countries. If a player, take Charles Piutau for example, is captured by one country, but has a passport for another country, they can apply to represent the country they have a passport for provided their last international appearance was more than 3 years ago. They will then need to play in an Olympics qualifying tournament, but can then represent their new country in any form of the game.
Cooper Vuna has already used this route to switch from Australia to Tonga.