MX - preloader setting width of mask clip?

Let me see if I can explain this in a way that makes sense...

I'm using the stardard preloader that expands the width of the loader bar in relation to the percent loaded. However, I am trying something a little different and it's not working.

I have an image that's under a mask. I wanted to use the loader bar as the mask to slowly reveal the image below as it expands.... it's not working.

The problem is that it doesn't seem to be stretching the loader mask (movie clip that is targeted to stretch). I know my pathing and everything is ok because I can remove the clip from the mask and it stretches just fine as the movie loads.

My guess is that Flash can't target a clip (instance name) if it is used in a mask.

Is this right, or does anybody have any suggestions?

I could be wrong but I don't think you can place script on a mask.