To be quite honest with you, this isn't possible AFAIK. A mask, by definition, is invisible. What you would need is to have the mask be the SHAPE you want, and then have an additional MovieClip that shows whatever visual elements you would have shown on the mask.
Case and point, if you wanted to use a glass-pane graphic as your mask, you would need to have a glass-pane graphic, the mask with the same dimensions, and then the MovieClip you're masking underneath that mask.
I hope that helps.