This is a rough, jury rigged idea.
If users, by convention, posted a PGP public key with the tag pgp-[username], other users could post pgp encoded messages to them with the tag message-username. Im sure the interface could be changed to ignore these tags unless theyre specifically searched for, but until then, posters could self-downvote to conceal their post from the trending board.
So for example
https://steemit.com/pgp-sigmajin/@sigmajin/this-is-my-pgp-public-key
Now, you could use the code to encode a PGP message and send it to me with the tag message-sigmajin, like so
https://steemit.com/message-sigmajin/@sigmajin/example-message
Now, i can see all the messages to me in order, here
https://steemit.com/created/message-sigmajin
I suspect it would be pretty easy to make a page where you typed the message, then the page looked for the recipients posted key, then encrypted it and posted it for you. To read your messages, you could paste your private key in a browser window and have a script decode them in real time on the browser side.
Also, it could concievably help with account recovery. Because the ability to decode a private msg in your messages tag would prove prior account ownership.
Wondering about opinions on this...