BuddyPress.org

a crazy idea – the BP_Activity class on the save method has another set of filters.

$this->content = apply_filters( 'bp_activity_content_before_save', $this->content, &$this );

Using $this
check for the activity type (new_blog_post),
grab the secondary_item_id which is the post_id (item_id is the blog_id here)
then lookup your custom excerpt in wp with the post_id and replace?