We have done something similar in our environment. However, we did not move the entire message.properties file - we created a custom_messages.properties file. To get CAS to look in the /etc/cas directory, we added the following to the cas.properties file.
cas.message-bundle.cache-seconds=0
cas.message-bundle.base-names=file:/cas/etc/cas/resources/messages/custom_messages,classpath:custom_messages,classpath:messages
You can, of course, set the file path to the specific directory you'd like to use.
-Dustin Luck
Pepperdine University