Jaakko Keränen updated the JHexen site with a progress report. He's currently debugging the new network code:
The networking model will be some kind of a semi-client/server where the clients exchange ticcmds through the server. It would be much more difficult to do a full client/server model: the clients would send ticcmds to the server, which would simulate all world events and then send the changed data to the clients. The hard part is figuring out what to send to each client...
Players won't be able to join a game when it's in progress for the same reason as demo recording needs to be started from the beginning of a level: the world must be kept in sync. I think things would get pretty complicated if all the changed world data would be sent to each joining player.
Still, lots more work needs to be done on his Doomsday engine (the 'basic' part of the engine) and the JHexen-specific code. A nice screenshot has also been posted, showing off per-pixel rendered fog with a Voodoo1 card.