Hi Jacob,
Sadly, there’s nothing available for the activity log. There may be a work around though, more on this in a bit. Other dashboard widgets sometimes have a filter or action you could use. For example, At a Glance offers the “dashboard_glance_items” filter where you can add/alter the items listed.
You can also add your own dashboard widget. These are actually meta boxes more than they are widgets. Review /wp-admin/includes/dashboard.php to see how these are implemented.
The activity widget is also defined there. You can see there are no immediate hooks available, but also that it calls wp_dashboard_recent_posts and comments. The query arguments for these are filtered. You could kludge some kind of messaging together through new custom post types, then include them in the queries.
