This is an idea whose time has come. Adi Robertson on virtual graffiti:
Imagine a world that’s filled with invisible graffiti. Open an app, point your phone at a wall, and blank brick or cement becomes a canvas. Create art with digital spraypaint and stencils, and an augmented reality system will permanently store its location and placement, creating the illusion of real street art.
Pop it up on Instagram, Facebook, SnapChat, or wherever. No paint required.