Of course, this requires that the aliens have precise knowledge of the distance to our solar system, and the Sun's motion through space. For an advanced society, that might not be too much to ask. But the kicker is this: they will know that, during the transit, our world is somewhere in front of the Sun's disk. So they could use a mirror array to focus their signal on that disk, thus reducing the power requirements for signaling to less than 10 watts, comparable to a bicycle headlamp! Yes, the mirror would now be a mile across, but it could be made up of a few small, cheap, and simple individual reflectors.

In other words, with a collection of mirrors, a small laser, and a computer to run it all, a knowledgeable and entrepreneurial extraterrestrial could produce detectable signals with only as much power as a handful of batteries could supply. No mammoth antennas, and no beefy transmitters are required. The broadcast could be an alien science fair experiment.

It's interesting to imagine that attempts by extraterrestrials to locate other intelligence in the Galaxy might be made not by officialdom in massive societal programs, but by the personal efforts of the young and the daring.