the kendoEditor() function hides the textarea and instead uses a table and an iframe, that's why it stops working for you as the textarea dom element is not visible anymore.
I don't think there is any easy way to make it work within the editor's iframe, as the iframe is like a different environment for your application.
P.S: This is the only thing I've been able to do with your sample http://jsfiddle.net/N4PVu/1/
as you will see, when the node drag starts I'm drawing an overlay over the iframe, so the iframe does not capture the mouse. I apply the kendoDropTarget to this overlay, and in the drop event I have set to "insert" a copy of the hint inside the iframe.