It would be better not edit the login page HTML itself if you don't have to, as it will be overwritten on updates. Instead you can use a configuration file setting web.conf
to change the message:
http://answers.splunk.com/questions/6096/custom-login-page-message-for-splunk-4-1-3
Yes, you can edit the default login page yourself and add anything you want to it. Be aware that any changes you make will be overwritten on upgrade, so make sure you add a step to your upgrade process to back it up and restore it each time.
The login page lives here $SPLUNK_HOME/share/splunk/search_mrsparkle/templates/account/login.html