I got a suggestion from someone via PM to just release whoever would end up in the wrong nation when I set the foreigner rules.
So say I set England no foreigners at all, I would release every single foreign player.
It is not a bad idea, but there is one caveat you need to keep in mind. Whenever you would release a player on a free day and the game gets updated with said player being transffered in the updated database, that transfer will go through. So this in a sense means you would end redoing, redoing and redoing your free transfers for all players that move during the transfer window.
Still, if I end up in a version where I am going to do most of the player movements, this is something I will keep in mind. So on that subject, thanks for the suggestion