Could a slight variation in this theme be used to designate disposable child keys for private messaging? 

As illustration, I DM someone a child key w/ validation which opens a private message channel where they reply to that validated child key with a message and a new child key & validation of their own and we continue replying — each time designating/validating a new key for reply. As the messages are encrypted to the users disposable keys, no one need know that the child keys exist or that we have continued our conversation.