Request new properties for the RadDockZone to allow the display of a message, image, or style when no docks are present in the zone along the lines of the EmptyMessage property of the RadInput control.  The properties could be:  "EmptyZoneMessage", "EmptyZoneImage" and "EmptyZoneCssClass".  See the following thread from 2010 where a solution to this was discussed:  http://www.telerik.com/forums/empty-raddockzone-message