I don't know how this can be done but a simple workaround could be to use the clipboard to store what you want and then paste it into the other app. Make a popup window (or a button)that will grab your current selection and convert it (if it's a listbox or a set of widgets) into text usable by the other app. Then for windows use Win32::Clipboard to fill the clipboard.
Comment on Re: Tk Drag and Drop Between Applications
Breaking user's clipboard will frustrate them and sometimes make them mad.
Also, there is an obvious place for race condition here: imagine two instances of same applications waiting for drag-drop from each other (using this d-n-d technique), clipboard will become invalid just on the half-way to a target.