What I have in mind is a solution that enables both streamer and streamed-to users being able to listen more or less synchronously, and have a chat via instant messenger at the same time. Does anyone know if this functionality is available anywhere (Mercora is Windows-only, so I can't use it)?

Mercora does have this functionality today. While we don't currently have a Mac version (or are you on Linux?), we're building a browser-based model which will allow us to support both platforms.

Thanks, Avikk. I look forward to the browser-based version (I'm on a Mac).