Please update the documentations for zk.Event:

http://www.zkoss.org/javadoc/latest/jsdoc/zk/Event.html#$init%28zk.Widget,%20_global_.String,%20zk.Object,%20_global_.Map,%20jq.Event%29

and state what types of objects are allowed for the 'data' parameter to the zk.Event constructor. It only seems to work with strings. I haven't tried a JSON object. It would be good if it worked with that also. In any case please put it in the documentation.