To be clear, when you say "mark+region", you're referring to the symbols like Pentagon, Clover, GB etc, that show where a mon came from, right?
Those are all governed by referencing the origin game data. Those things are not data by itself.
(Also if you're referring to the line met in johto [for example], that line is also based on the origin game)
it would be? because such a mon is not legal to begin with?
Like I said, origin game persists. So, for example, if Yveltal was caught in Pokemon X, that is illegal.
Just because the game doesn't know it's illegal, doesn't mean it isn't.