How to manipulate the list of a Struts2 <sj:select /> before it is loaded?

I have implemented Struts2-jQuery <sj:select/>.

This select box is populated fetching a JSON object coming from an AJAX call.

How can I access this JSON object before the loading of the SelectBox in javascript in order to do some processing on it?

The Struts2-jQuery-plugin's SelectTag has an attribute for that:

onBeforeTopic : Topics that are published before a load

Example of usage:

<script type="text/javascript">
    $(function(){
        $.subscribe("myBeforeHandler", function(event, data) {
            alert("I'm raised before the loading of the SelectBox ! Data is : " + data);
        });
    });
</script>

<s:url var="myUrl" action="foobar" namespace="/" />

<sj:select onBeforeTopics = "myBeforeHandler"
                     href = "%{myUrl}"
                     list = "foo"
                     name = "bar" />