Progress.
I have PubNub working in publish-subscribe mode in a rough and tumble way between two unix.com user accounts in two different browsers (Safari and Chrome) using two different URLs (one on the main site and one in local host).
Now, I just need to work on how to set up various publish and subscribe chat services on a user-to-user basis, permissions, subscriptions (join and unsubscribe), archiving the chats, etc. In other words, I have a long way to go to get something working that could replace the current PM system and be really cool, but I'm making progress.
With what I have already, I could build a member chat room for all users and guests (no permissions, no subscriptions, just a single chat room, using what I have scaffolded together right now; and I might create a single room for fun (and for testing) before moving on to more complex tasks.
I'm using the PubNub API and we get up to 1 Million transactions a month free, so we should be OK since the site is not very busy these days