Normally properties list display and set values of Workflow Node object, not a content object on which you start a workflow. Contant is just attached as children of bpmackage.
One can try to display them in a panel that lists content, attached to workflow. Try modifying alfresco\jsp\workflow\start-workflow-wizard\workflow-options.jsp (For example, look at adding columns to <a:richList id="resources-list" …)