I will be adding functionality to all the controls that allow for easier development of client-side events in the future.  I just don't have the cycles to do so right now.  However, I can offer you this script to allow you to accomplish your goal.

	initTree();
	function initTree()
	{
		var oTree = dnn.controls.controls['MyDNNTree'];
		for (var i=0; i<oTree.rootNode.childNodeCount(); i++)
			attachHandler(oTree.rootNode.childNodes(i), oTree);
	}
	
	function attachHandler(oNode, oTree)
	{
		var oTNode = new dnn.controls.DNNTreeNode(oNode);
		var oImg = oTree.getChildControl(oTNode.id, 'expcol');
		if (oImg != null)
		{
			dnn.dom.attachEvent(oImg, 'onclick', expandCollapse);
			for (var i=0; i<oTNode.node.childNodeCount(); i++)
				attachHandler(oTNode.node.childNodes(i), oTree);
		}
	}
	
	function expandCollapse()
	{
		var oImg = dnn.dom.event.srcElement;
		var oTree = dnn.controls.controls['MyDNNTree'];
		var oNode = oTree.DOM.findNode('n', 'id', oImg.nodeid);
		var oTNode = new dnn.controls.DNNTreeNode(oNode);
		alert(oTNode.id);
		return true;
	}

Please note that this is much more complicated (i.e. more code) than I intend it to be.  I would look at this as a temporary workaround until I provide you a cleaner way to accomplish the same thing.  This script also assumes that when the initTree method is called all nodes that need to sync the event are present (i.e. no POD)...   you can modify the script as needed if you require POD.  I would probably use the status function capability to attach the events to the newly retrieved nodes.