You can use BitmapDatafor this.
class members:
private var reflect:Matrix = new Matrix(1, 0, 0, -1, 0, 100);
private var reflectionData:BitmapData = new BitmapData(100, 100, true, 0);
private var reflection:Bitmap = new Bitmap(reflectionData);
INIT:
reflectionData.draw(canvas, reflect);
reflection.x = canvas.x;
reflection.y = canvas.y + canvas.height;
addChild(reflection);
in animation / redrawing
reflectionData.fillRect(reflectionData.rect, 0);
reflectionData.draw(canvas, reflect);