I was wondering why you did not use morphs for mondrian since I have the impression that for drag and drop alex is reinventing the morph event dispatch loop.
I don't know exactly the current situation, but when I last worked on Mondrian for Squeak it had drag and drop. These were a couple of lines of code built on top of the basic mouse events. Essentially this is the same as when you implement drag and drop in the HTML DOM tree.
Morphic is way too slow, way to wasteful with memory and too platform dependent for something like Mondrian. One of the reasons why the original Mondrian is not portable, is because it is tightly connected with the graphic system of VisualWorks.
Lukas