I am not sure if this is quite what you are looking for but here is a solution I found for adjusting alpha on a TextField object.
the key is to set the blendMode property on the sprite to LAYER.
// this is obviously not php code. I thought it would help for legibility
// create an empty sprite
var rect:Sprite = new Sprite();
rect.blendMode = BlendMode.LAYER;
var txtField:TextField = new TextField();
txtField.txtColor = 0x000000;
txtField.alpha = .5;
Hope this helps!