We ended up adding a client side JavaScript function to enforce the comments in the jsp page (checkin_step.jsp) and added it to the definition of the Checkin step (in custom-actions.xml) so that this function will be called when user checks it in.
<script type="text/javascript">
function validateCheckinComments(){
<%boolean isDoc=wt.doc.WTDocument.class.isAssignableFrom(obj.getClass());
if( isDoc){
%>
var comments = getElementByName('comment');
if( comments == null || comments.value == null || (comments.value=comments.value.replace(/^\s+|\s+$/g,")).length==0){
alert('Comments are required to checkin a document.');
return false;
}
<% }
} %>
return true;
}
</script>
<action name="checkin_step" ajax="row" afterjs="validateCheckinComments">
Raju Pulavarthi
