Sadly, you can't really draw text over a progress bar or any other graphics without some sort of complication. Viewports aren't drawn transparently so any text drawn over a graphic will blank out where the text viewport is, and any graphics drawn over text will hide the text under it. It's best to try not to overlap viewports because of this.
However, if you really want to have text show over an image, make sure the image is drawn to the screen first, then the text. This is done by having the image display tag (or in your case, the progress bar tag) before the text you want to have over the image.
I hope that helps some.