>Planets are very poor reflectors. Not comparable.
Not comparable!??? From wikipedia:
"The average overall albedo of Earth, its planetary albedo, is 30 to 35%, because of the covering by clouds, but varies widely locally across the surface, depending on the geological and environmental features."
Just because one square inch of mirror reflects better than one square inch of planet surface does not mean the mirror will be more visible. Your positionable mirrors will still need to cover a surface area that's a pretty large fraction of a planet.
BUT unfortunately that's not how Kepler detects planets. Assuming the detectors can detect an increase in brightness as well as a decrease...you're going to need an array that's close to 100% reflective and exactly the size of a planet.
>What you do is point at a target, let fly, then point at the next target, let fly, etc
From wikipedia:
"While only about a dozen planets have been confirmed in the habitable zone, the Kepler spacecraft has identified a further 54 candidates and current estimates indicate that there are "at least 500 million" such planets in the Milky Way."
Either the transmitter or receiver will need to have a wide lobe...otherwise the probability of intercept is stupid low. If you could position your absolutely massive mirrors array at a rate of 1500 planets/second, you would, on your receiving planet, see a signal once every three days for 1/1500 of a second. Kepler can see something like 15 degrees, and requires DAYS of averaging to get something statistically useful. Seeing a 1/1500 second signal even with, literally, planet sized light blockers or reflectors isn't anywhere close to our grasp yet.
Not saying that some aliens aren't doing this....but I don't think we're ready to see it yet.
And, if you're talking about planet sized mirror arrays being feasible, why not planet sized light blockers arrays that can be modulated? Way easier to construct. Is there a benefit in making the average brightness of a sun look brighter rather than darker?