BuddyPress.org

Create a post category,
Place notification posts in that category,
Place a loop that queries only that category,
Set the amount of posts to display to one,
Place that anywhere in your template files you want the notifications.
Customize the look and what the post displays easily by using ( the_content(), the_excerpt(), the_thumbnail(), title or whatever ect ) to style it the way you want.
Easy way to that.