Philipp15b

This plugin allows you to add your code/text to the headers and footers on all the blogs in the blog network and in the backend of all blogs too.
In addition, it is possible to replace the footer bar text in the backend completely.
You can access the settings under Network Admin -> Settings.

Sitewide (Admin) Header/Footer

Change the header/footer of the front- and backend of WordPress Multisite

Jennifer M. Dodd

The BuddyPress default theme provides users with an area in the sidebar for the 
display and dismissal of sitewide notices. This plugin wraps that code in a 
widget so that users can easily add that functionality to their own widget-ready 
sidebar without editing <code>sidebar.php</code>.
<h3>Credits</h3>
Development funded, in part, by Ariel Meadow Stallings and the Offbeat Empire.

BuddyPress Sitewide Notice Widget

Wrap the BuddyPress sitewide notice in a widget.

Sitewide (Admin) Header/Footer vs BuddyPress Sitewide Notice Widget