BuddyPress.org

I was looking around for something similar. Not exactly what you are talking about though. I ran across:

wp-o-matic which aggregates rss feeds to a blog

http://devthought.com/wp-o-matic-the-wordpress-rss-agreggator/

donncha’s site wide tags (which doesn’t work with bp at the moment)

http://ocaoimh.ie/2008/10/01/wordpress-mu-sitewide-tags-pages-031/

Any solution you find will probably not include your requirement for being selective about the posts you show. You are probably going to have to find something near what you want and modify it for your needs.

Maybe somebody else has a different approach or suggestion?