<script type="text/javascript" charset="utf-8">  
    $(document).on('ready pjax:scriptcomplete',function(){
 
    // Identify this question
    var thisQuestion = $('#question{QID}');
 
    // Insert header text
    var insertedText = "Inserted text";
    $('table.subquestion-list thead tr:last td:first', thisQuestion).addClass('inserted-header').append(insertedText);
  });
</script>

td.inserted-header {
  font-weight: bold;
  text-align: center;
  vertical-align: middle !important;
}