@lautering
I had a similar dilemma but managed to figure it out, even how to add javascript without screwing up the activity stream (see https://buddypress.org/forums/topic/add-content-above-activity-stream-in-bp12 ).
Assuming you’ve elected to have site activity shown on your home page, you want to add content to bp-default>activity>index.php. Not home.php. The neat thing is that you can add content that displays to non-logged-in users that is different than that displayed to logged-in users.
You can see it in action on my site here: http://rspace.org
