I see from the demo site as well as the issue queue that I can set up a Views-based stream that takes a node ID as an argument and would display activity messages for that node. Display of attachments is apparently not possible.

I’m not sure this is precisely what I’m trying to achieve. What I’m trying to achieve is basically a activity wall for pieces of content (nodes) that only shows messages posted on that node page. It will be like comments, except with features that are more common now on other sites, such as liking comments (flags), Ajax display, avatars, and the flexibility to add other attachments.

Would this be best accomplished by Heartbeat? If so, is it as simple as adding a shoutbox to the content panel template, as well as a stream display created by a simple module using the Heartbeat API? Is the heartbeat_example.module a good starting point for that?