Actually, this seems like a pretty good idea.

I presume that if G+ really is a collection of smaller projects, then this functionality would sit neatly inside the stream area.

It's also one of those ideas that once you hear, seems obvious - so wouldn't be too surprised if they're already working on this. It'd only really be useful if/when there is a large amount of newsfeed traffic on there, similar to how busy facebook is for most of us now.