JSF: Exibir um popup durante a execução de um evento

<a4j:commandButton
status="waitModalStatus"
title="Enviar arquivo via FTP"
actionListener="#{processoMB.sendFtp(upload)}"
image="/resources/images/16x16/up.png">

<rich:popupPanel
    id="waitModal" style="text-align:center" autosized="true" modal="true" width="200" >
<h:graphicImage library="images" name="ajaxloader32.gif" alt="ai"/><br />
    <h:outputText value="Transmitindo o arquivo, aguarde!" />
</rich:popupPanel>

<a4j:status name="waitModalStatus">
    <f:facet name="start">
        <rich:componentControl event="start" operation="show" target="waitModal" />
    </f:facet>  
    <f:facet name="stop">
        <rich:componentControl event="stop" operation="hide" target="waitModal" />
    </f:facet>
</a4j:status>