jQuery code injection will do the job. Just inject the following code on your form page, replacing the category and action with your own labels.
<script type="text/javascript" src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function(){
$('input.button').click(function() {
window.setInterval(foo, 100);
});
});
function foo(){
if($(".form-submission-text").is(':visible')){
_gaq.push(['_trackEvent', 'YourForm', 'Submission']);
}
}
</script>