Yes!! Put the following in the configuration file web.conf (perhaps in etc/system/local or etc/apps/search/local)
login_content = <BODY><div style="padding:5px; border: 1px solid #ccc;"><b>Your message here</b></div></BODY>
You don't have to follow this style exactly - I just wanted to show that formatting is possible...
Here is what the docs say
login_content = <content_string>
* Add custom content to the login page
* Supports any text including html
And you can find out more by looking at web.conf.spec
