The desired behavior cannot be achieved out of the box with the content provider from this KB article, because the paths to the images are passed to a generic handler.
The main reason it can't be achieved at the moment is that the latest moment of the page life-cycle of the FileBrowser.ascx dialog, when the user can set the FileExplorer's InitialPath is before the Page_Load event, but the path to the preselected in the Editor item is passed in the Page_Load event and the initial path is reset internally.