Dan, Here is what I did to use a graphic (Spark Primitive Graphic).
<fx:Declarations>
<s:Graphic id="mapOverlay">
<s:Group>
<s:Path id="mapOverlayPath" >
<s:fill>
<mx:SolidColor id="ovlayColor" color="0x000000" alpha="0.8"/>
</s:fill>
</s:Path>
</s:Group>
</s:Graphic>
</fx:Declarations>
//This code called from some function
mapOverlayPath.data = "M 0 0 v " + map.height + " h " + map.width + " v -" + map.height + " v -" + map.width + " M 100 100";
mapOverlayPath.data += " v " + (map.height - 200) + " h " + (map.width - 200) + " v -" + (map.height - 200) + " Z";
map.staticLayer.addElement(mapOverlay);