By following this drag&drop example it should be possible to implement delayed request for drag & drop data: http://qt-project.org/doc/qt-4.8/draganddrop-delayedencoding.html

However this fails on OS X where the clipboard data is requested immediately once the drag & drop event is started. This makes it impossible to delay heavy data operations to the point where the object is actually dropped on the target.
This works correctly at least on Windows.