I think best practice is to override the navigation-rule about login/logout/relogin actions in faces-config-custom.xml, so that the login page relies in (for example) jsp/extension/custom-login.jsp
then, you can do anything in your custom-login.jsp to make it suite your needs.
I say "best practice" because then you won't edit a "native" Alfresco file (jsp file here), and will be able to get your hands back on it in the future, if needed.
good luck