To anyone else that runs into this, the workaround I've come to is to use an event listener for the text field's Event.CHANGE then in that handler add an event listener for Event.EXIT_FRAME. In the exit frame handler, the TransformManager selection controls can be updated (since by that time the text field has auto resized itself to its new text content). Although calling updateSelection on the TransformManager seems to work at first, when you click and drag a handle to resize the text field, the size jumps to a different size. So instead, I ask the TransformManager to deselect its selection and then re-select it. The Event.EXIT_FRAME handler then removes itself as a listener so that it is only called once. Not pretty, but it seems to work. Let me know if someone has a cleaner solution.
Ben