What about this way. You sign up as a mercenary if you want. Meaning, every time you log in you are automatically put in the country with the lowest numbers. If there were enough mercenaries, that should maintain a reasonable balance eliminating ENY restrictions. My WAG is about 20% would sign up for mercenary duty. Twenty percent might not be enough.
Another thought would be the mercenary would be in the country with the lowest average rank.
And Mercenaries don't earn country win perks.