I wanted to hit a planeswalker and the creature it created, but this notably hits Fable of the Mirror Breaker, Wedding Announcement, Gruff Triplets, and some other non-walkers.
I'm pretty sure this works under the existing rules and you can "remember" a token's creator, but not 100%.
This is a neat idea. I think it would function as an Alchemy card, where the software could keep track of what had created the token -- in fact, I wouldn't be surprised if it already does that. But yeah, for paper play, it would be a nightmare. Was the Mite you're targeting created by this Mirrex, or by that other Mirrex that got Field-of-Ruined a couple of turns ago?
Yeah you're right, it's just too much overhead. The examples below create the tracking requirement when those cards are played, this would require that you always track every token from the start of the game forever.
Could definitely work in Alchemy though.