This is a continuation of the question below.
Basically I discovered its no problem at all to draw lines or shapes or whatever directly onto an mx:Text object using its graphics property (Text.graphics.lineTo, etc.). If just displaying that text, then any such drawn lines are displayed as well. However, if that Text object is used as a mask, then any drawn lines on that text object are merely ignored when the mask is rendered.
And its hard for me to see the reason behind this, if anyone else is able to. I mean you're designating the Text object itself as the mask. It seems like any visual change to that text object should be part of the mask.