Yeah, all the list widgets are "convenience widgets", they are not designed for general flexibility. You probably would find the Tk::Canvas will do what you want, and you can make your own custom widget, which is based on a Canvas. See Tk::CanvasDirTree for an example of subclassing a canvas to a custom widget.

However, one of the list experts may come along and find a hack for you.... it takes alot of experimenting with the browsecmd, etc..... and it may be impossible as you say.