Multisite Widget/Content

Could you use the admin bar at the top instead? To me short messages look better up there.

add_action( 'admin_bar_menu', 'toolbar_link_to_mypage', 999 );

function toolbar_link_to_mypage( $wp_admin_bar ) {
  $args = array(
      'id'    => 'my_admin_bar_text',
      'title' => 'WELCOME TO SYDNEY',
      'meta'  => array( 'class' => 'my-toolbar-page' )
  );
  $wp_admin_bar->add_node( $args );
}   

Likewise though, you can create a short message on the sidebar, but it would have to remain pretty short. The code to do that would be:

add_action( 'admin_menu', 'register_my_custom_menu_page' );

function register_my_custom_menu_page(){
  add_menu_page( 'custom menu title', 'custom menu', 'manage_options', 'custompage', 'my_custom_menu_page', plugins_url( 'myplugin/images/icon.png' ), 6 ); 
}

function my_custom_menu_page(){
  echo "Admin Page Test";   
}

This will add a “custom menu” link to the left (just under Posts). You could name “custom menu” whatever you want, and it has the added bonus of creating a page that you can put more information on if you’d like.

Both of these options, when enabled on the network site, they can’t be disabled per blog. If not network enabled, then it can be enabled and disabled per each blog BY each blog admin.

note: the first code snippet above, I don’t remember where I got it, it’s been chopped around for a while. The second snippet is from WP Codex, http://codex.wordpress.org/Function_Reference/add_menu_page