If it's that important and the OP hasn't participated in that answer yet and you think the OP won't notice it when reading answers later (but if he doesn't care that much...), then you should comment on the answer and the question. Delete one of those comments later. All three of these conditions combined should be really rare.
I only want to check one place to see who will match, especially on questions with several answers (say 7-9). Not the current question and somewhere else, even if that is only the OP's name. You'd have to determine a sort order, and it's just not worth the complication. Keep it simple. If you must ping them, you know how to do it: comment on the question.
Stack Exchange isn't instant messaging, even if it's important we can reply sometimes.