[BlooDHounD]

package {

	import flash.display.Sprite;
	import flash.events.Event;
	import flash.display.DisplayObject;

	public class TopSprite extends Sprite {

		public function TopSprite() {
			super();
			super.addEventListener(Event.ADDED_TO_STAGE, this.handler_addedToStage);
			super.addEventListener(Event.REMOVED_FROM_STAGE, this.handler_removedFromStage);
		}

		private function handler_addedToStage(event:Event):void {
			super.addEventListener(Event.ADDED, this.handler_added);
		}

		private function handler_removedFromStage(event:Event):void {
			super.removeEventListener(Event.ADDED, this.handler_added);
		}

		private function handler_added(event:Event):void {
			if (event.target === this || ( event.target as DisplayObject ).parent !== super.parent ) {
				super.parent.setChildIndex( this, super.parent.numChildren-1 );
			}
		}

	}

}

вроде так. не проверял