Are you wanting to use this, or does this just fall in the category of "It would be cool if..."
We would prolly want to use this. Imagine a property editor, that, when clicking on a property to edit, changes into an apropertiate editing widget. Combined with <interfaces> this is extremely powerful. Marking zopestudio and adding to CC-list.
This is awesome. You could now, theoretically, create a cascading tree view of some document, edit it, then swap the classes to view it in "preview" mode. Might even help with creating a xul editor.