rich-text-pane drop callback

I want to intercept drop events going to a rich-text-pane. Does anybody know how to do this?

I've tried specifying :drop-callback as an initarg and I've trying (setf (capi:simple-pane-drop-callback self) 'func) just before showing the pane, but neither of these seem to do trick (the rich-text-pane simply accepts the drop events and my code is not visited).

thanx