Mobile shader for 3D drawing app (Unity 3D)

I'm looking for someone who can write a shader with eraser functionality. The idea is that I'll use one object (circle, square) which will move over other 3D objects and erase the parts it overlaps (goes over another object), just like an erase in any drawing app would. There will be multiple 3D objects (quads) with textures applied and I want parts of those textures to become hidden. Additionally, I'd like to know how much of the texture was erased (using shader properties), so I know when it fully visible or fully erased. Ideally, there will be an option to set the hardness of the eraser, so the erased edges are smooth (see attached picture). (Optional) There will be smudges left where the object got erased (maybe on the background?). The shader needs to be optimized for mobile devices (there will be multiple partially erased objects on the screen and it should still run on iPhone 4S). One 3D object will be an eraser and other 3D objects (quads with a single texture on them) will get erased. The shader should work on iOS and Android. I can provide an example scene if needed. Please provide a realistic budget and timeframe.