The beauty of this idea is that it is a passive signal requiring no broadcast energy or targeting. And, it is detectable over a sizeable volume of the Galaxy. Nor does the billboard require any long-term commitment for maintenance by the host civilization. Set it up and forget it. If the home planet's space budget dries up, so what? In principle a space shade could be designed to last for millennia and even outlast the lifespan of its civilization.
An alternative approach proposed by Arnold is to have a freight train of objects successively transit the star sequentially, and in a distinguishable manner. "At each period, we would observe a series of transits whose number and timing would claim its artificial nature and a will to communicate," he wrote. For example the sequential transit could communicate a series of prime numbers or a binary code.
The billboard would need to be placed close to the star, far inside its habitable zone, so that successive transit could be seen every few days.
Could there be at least once super-civilization within Kepler's field of view that is winking back at us?
Images: Luc F. A. Arnold/Observatoire de Haute-Provence in Paris