ActionScript 3.0 :: How To "invert" A Mask

Normally a mask reveals whatever is within the layer below that is overlapped by whatever is on the mask layer. I'm trying to do the opposite. What I'm working on starts with a city street grid. I have an irregularly shaped mask that "grows" from the center, revealing some wireframe city buildings. At the same time I need to remove the portion of the street grid that is underneath the buildings, using the same irregular mask (well, a copy of it, but identical motion/position, growing out from the center). The relevant portion of the street grid is its own movieclip/layer - easy to mask or otherwise manage by itself.

It looks like there is no easy "invert mask" function (and why not, I ask - seems like it'd be pretty straightforward and certainly useful).I'm thinking I could start with a big rectangle and use my mask (at tiny "starting size") to punch a hole in it. Then I could scale the mask until the hole overlapped the street grid completely.

But the problem with that is, then I have this huge mask, and the street grid/city movieclip (which contains all the clips and masks that make up the buildings and street grid) resizes and repositions dynamically to fit the browser window size. The huge mask will change the size of the grid/city movieclip, which will change the way it resizes (basically, Actioscript will now think it's huge so it'll shrink it way down). Is there a way to have Actionscript "ignore" the size of the mask when determining the size and position of the movieclip?Or, perhaps there is a way to resize just the punched-out hole in the middle of the mask, and keep the outer rectangle the same size?How do people usually do this sort of thing - when they need to make something disappear via a "growing" mask?