You better just use it the following way (put a condition on the disabled attribute)
<p:panel>
<h:outputText value="Project Title"/>
<p:inputText disabled="#{not projectBean.updateFlag}"
value="#{projectBean.selectedProjectDo.projectTitle}" />
<p:commandButton disabled="#{not projectBean.updateFlag}" value="Update"
actionListener="#{projectBean.updateProject}" />
<p:commandButton value="Cancel" actionListener="#{projectBean.cancelUpdate}" />
</p:panel>
In general : don't use JSTL tags unless you really need them...