You should be able to add the custom message on the login page by overriding the include home_page_static_content in WebCenter Content.
Refer the following articles to develop a custom component to achieve your requirement.
How Do You Customize the WebCenter Content Login Page? (Doc ID 1309758.1)
How to remove all links from Webcenter Content landing page except Login (Doc ID 2284587.1)
The earlier provided references are the ones to be used for WCC for any UI customization. Along with that you can review the following documentation : https://docs.oracle.com/cd/E23943_01/doc.1111/e10807/c03_look_nav.htm#CSSDK1404