What about using a tag dm-username for implementing rudimentary public non-encrypted direct messaging? Then you can just use the tag system to find messages directed to you.
Another convention you could use is create a thread specifically for direct messaging you, or combine the two with the tag #dm or dm-username on a post signaling that you can use that thread to send messages.
I am really happy about the OpenSeed work and I think that might work even better over time, but I think having a public Direct Messages convention could hugely improve the way we deal with issues here. Without that Hive would be basically less capable than Twitter in a significant way.