Oddbean new post about login | logout I agree. You would basically have it monitor (every?) note and check which relays it’s on some interval for the first X minutes of its life. It’s quite challenging though because it happens fast!
I think you could just sample and experiment with sending a singular test note to a small relay. But yes, you would have to subscribe to a boatload of relays at the same time right before sending the note, filtering for the note that was just sent.