BitmapData.fillRect is apparently not implemented.  The following haxe program demonstrates the issue:

 import flash.MovieClip;
 import flash.display.BitmapData;
 import flash.geom.Rectangle;

 class Main {
     public static inline var RECT_SIZE : Int = 25;

     public static function main() {
         var root : MovieClip = flash.Lib.current;
         var mc1 : MovieClip = root.createEmptyMovieClip("mc1", root.getNextHighestDepth());
         var b1 : BitmapData = new BitmapData(RECT_SIZE, RECT_SIZE, false, 0xffff0000);
         var mc2 : MovieClip = root.createEmptyMovieClip("mc2", root.getNextHighestDepth());
         var b2 : BitmapData = new BitmapData(RECT_SIZE, RECT_SIZE, true, 0xffff0000);
         b1.fillRect(b1.rectangle, 0x80000000);
         b2.fillRect(b2.rectangle, 0x80000000);
         mc1.attachBitmap(b1, 1);
         mc2.attachBitmap(b2, 1);
         mc1._x = 0;
         mc1._y = 0;
         mc2._x = RECT_SIZE;
         mc2._y = RECT_SIZE;
     }
 }

Expected result:

 The movie should show a solid black square in the upper left corner, and diagonally down from it a 50% transparent black square.

Actual result:

 The movie shows two solid red squares.

I've attached a screenshot showing the rendered results in Adobe Flash (on the left), and in swfdec (on the right).