We can add some hover events for the next release. Now you can handle the MouseMove event raised by the base Control class to find the node under the mouse pointer:

var mousePos = e.GetPosition(diagram.DocumentPlane);
var node = diagram.GetNodeAt(mousePos); 

If you only need to change the appearance of a node when the mouse is over it, you can use a style that defines setters for the MouseOver state inside a MouseStates VisualStateGroup. There is a sample style defining selection states at the end of this topic, the ones for the mouse should look similar:
http://mindfusion.eu/onlinehelp/diagramlite/index.htm?Styles_and_Control_Templat...

I hope that helps,
Stoyan