He encontrado esto... no sé si servirá
Código:
var format:TextFormat = new TextFormat();
format.align = "justify";
var txtField:TextField = this.createTextField("txtField", this.getNextHighestDepth(), 100, 100, 300, 100);
txtField.multiline = true;
txtField.wordWrap = true;
txtField.border = true;
txtField.text = "When this text is justified, it will be "
+ "spread out to more cleanly fill the horizontal "
+ "space for each line. This can be considered an "
+ "improvement over regular left-aligned text that "
+ "will simply wrap and do no more.";
txtField.setTextFormat(format);