Isn't the ["-"] tag be sufficient for topical relays to filter out rebroadcasts? It doesn't prevent generic free-for-all relays from accepting topical notes, but it surely allows topical relays to be selective about what it will accept according to the user desire. 

 - doesn’t signal anything, this is like sending and email with To: -. This doesn’t mean anything. Id be happy with a p tag with relay pubkeys.