Painting a mask with a non-circular brush

I need for the user to be able to unveil a hidden image by painting it - á la the classic scratch lottery ticket. I can get it to work by placing the hidden image on top of the background image, putting a mask on it and letting the user paint the mask with drawCircle. But I would like for the brush to have a more natural feel.

Is there any easy way to let the user paint the mask with a brush that gets its shape from a bitmap, or just to make it more natural some way? (Having an oddly shaped bitmap follow the user’s cursor around while painting doesn’t seem to work - the bitmap gets converted into a rectangular shape when placed in the mask clip.)

I made an example a long time ago demonstrating something like this:

I don’t know what version of Flash youre using, but this is AS2, so if you want to see just the code, I can dig that out for you - maybe even up convert to AS3 if I have time.

This uses a “stamp” approach - think of it like a PS brush with spacing (more easily seen with the “/” shape). For something smoother, you could iterate through multiple stamps (based on distance) between individual mouse positions.