I have been twisting my mind around this all morning. I know there are several ways to make textures become an outline and I also could just do it letter by letter but I think there is a way to make it so as I type each letter another box appears. Well enough talking, let me show you what I mean.
I would think you could make this work by using a glow effect that was cropped at a certain point. Might that work? You could just use an invisible border to create the bounding box, and then use a colored glow to fill in the area around the letter inside of the box.