This may not be a supported method, but the file that generates the login page can be found here:
$SPLUNK_HOME/share/splunk/search_mrsparkle/templates/account/login.html
If you just want to change the title bar text, you should be able to change the line that looks like this:
title = _('Login')
To something like this:
title = _('Server Junkyard Login')
