Old thread, but i wanted to update since i worked on this problem today, i didnt have jquery in my project so i did it the plain old javascript way, i also needed to block the content on the screen so in my xhtml
<img id="loading" src="#{request.contextPath}/images/spinner.gif" style="display: none;"/>
in my javascript
document.getElementsByClassName('myclass').style.opacity = '0.7'
document.getElementById('loading').style.display = "block";