This is interesting, and is a possible way to embed messages inside anonymous payments, at the cost of transaction fees.
So if a payment contains a No-op string that says “Message encrypted for Public Key [xxx]: [yyyyy]” then that gets passed along to the destination? Or even “Cleartext message to recipient: [zzzzz].”
Of course the content of these messages - the [xxx] or [yyyyy] have nothing to do with Bitcoin, but they could be used as part of a layer on top of Bitcoin.
The reason I’m so interested in embedding messages is that they allow use of a “static” anonymity network like Freenet, rather than a “live” network like Tor or I2P. Live networks have exit nodes, a few of which can be compromised. If a government compromises 1% of all exit nodes, then they have a small chance of figuring out where a given site is hosted. So to use my Heroin Store example, they would send an N byte message to the store and watch all of their compromised exit nodes for N byte messages. They’d do this over a few hours, watching where N byte messages got sent to, eventually discovering the IP address of the store.
In a network like Freenet, data just floats around - there is no one machine where a website (for example) lives. You publish data to the cloud, and as long as people access it periodically, it stays around.