With the approach based on pub/sub channels, this is possible to do if you know the name of the session (i.e. know the name of the channel).

Of course the hard bit then is; how does the client know there's new information from the agent, or a new session?

Generally we'd recommend having a separate kind of 'notification' or 'control' pub/sub channel that clients always subscribe to to be notified of new 'sessions'. Then they can subscribe to the new session based purely on knowing the session name.