It appears that portal chat won't work in a cluster without setting portalchat.cluster.channel in sakai.properties. But this property is not documented. In addition to putting it in the documentation, I'd appreciate some notes here, since I may not be able to proceed with testing until I've set it up properly.
If valid use of portal chat is going to require enabling multicast on all servers, or doing some other configuration of jgroup, we're going to need a special configuration note for the portal chat, with sample configurations for likely setups. Note that our experience with jgroup in jforum has been bad. I trust something different has been done this time.