Can't speak for HN but chat, like video comments, seems to prove difficult for many communities. Such features can be added but whether people use them or they fall into non-stop trollery is anyone's guess.
#startups on irc.freenode.net is reasonably active though and would usually appreciate a discussion on topics coming up on HN :-)
Sounds like a good idea. However the upvote would have to have a time limit, like upvoted in the last 24 hours. Otherwise powerful arguments will own the top 10 forever :) Essentially, it would look like ASK HN, live.
Basically, each top story would have its own chat room. In the landing page of the app, I can see each room and the number of participants.
Later, I would love to be able to upvote pieces of chat so that people can easily find relevant discussions in the chat.