Every caption you see on the login screen comes from a file that has all messages listed in it. It is somewhere in /opt/zimbra/tomcat/zimbra/. What you should do is find a more or less unique phrase from that login screen and run
Code:
grep "your phrase" /opt/zimbra/tomcat/zimbra/* -R
You will get a couple of files that hold that phrase. One of them will be the one that controls all the text in the entire web client. So you simply modify that file and get you need.
Good luck