BuddyPress.org

I don’t think site wide activity would work if you’re using a fresh, external install of bbPress.

That would need to be coded probably as a bbPress plugin. I highly recommend deep integration between WPMU and bbPress so you can gain access to WP’s function within bbPress. Otherwise, you need to do specific calls to the database.

If you’re going with the latter approach, for hints, look into how Boone Gorges coded the MediaWiki plugin, which also writes to the sitewide activity stream.