http://hubski.com/tag?id=politics A couple of redditors recently tried something somewhat like that with wubel.com http://www.wubel.com/channel/technology We've talked about the right-hand side for discovery, but like you said, it's kind of a force-feed. That's why I am kind of leaning towards users electing to have some portion of their feed be non-followed content. It would be obvious which posts were coming from the outside, and you could toggle how much of it you wanted to see.
Another idea is to highlight active tags. Clicking on a tag to see a mostly empty page discourages further clicking on tags. It might be worth trying out to have random stuff show up on user feeds. Make the background a slightly different shade to distinguish it. One key is to be able to dismiss such stories from the stream. I can tolerate annoying stories if I am able to hide them.
That's my current thinking. If I could see them for what they are, and dial it up or down, then it might be a good approach. Feed feeling stale? Dial up the outside content component. Too much noise? Dial it down.