I don't know that Mat isn't a Hero Reborn, but we know that all of his memories can't come from his "Past Lives." This is a fact, because when Asmodean is still playing at bard in book 4, he is singing a song about a Manetheren battle, and Mat's memory is twinged. He remembers playing two seperate roles in the battle (the general, and some other guy). I don't think the Pattern spins out people more than one at a time.