This can be done using the button_bar.vm template in the look project.
To add a site wide broadcast, you need to check out the branding repository from the look project. Add a templates'directory and the path into the button_bar.vm file.
The button_bar.vm template can be downloaded from the sample branding bundle available at:
http://collab.net/community/cif/csfe/51/BrandingBundle-Basic.zip or http://collab.net/community/cif/csfe/51/BrandingBundle-Basic.tar.gz
<p><font color=#ff0000>Site Maintenance Planned</font></p> pfont color=#ff0000>Site Maintenance Planned/font>/p>Add your message before the <form> tag at the top, and commit the changes to the repository.