Jonathan,

By using the existing JavaScript that Laserfiche has near the bottom of the Login.aspx page I was able to add in the placeholder text for the username element.

<script type="text/javascript">
/* Focus on the user text box if not disabled */
(function () { var a = document.getElementById("TheLoginInterface_UserBox"); if (!a.isDisabled) { setTimeout(function () { a.focus() }, 250); a.placeholder = 'domain\\username'} 
})();
</script>