Stenciling on Unity iPhone

Greetings all!

Once again I am stumped on how to translate a technique using Unity iPhone. Basically I have a game where the user will swipe the screen and the idea is that where they swipe will 'reveal' the texture underneath. Think of it like a scratch ticket for the lotto or a flash light.

Normally I would make something like this by accessing the stencil buffer, or writing a specific alpha shader. Does anyone have advice on an equivalent approach for the iphone? Is there a way to draw a brush into an alpha texture and use that as a stencil mask ?

Use a drawing script (unifycommunity wiki) to draw on a Texture2D. Then use that on a plane and place it in front of the camera. A bit dirty, but it might work.