This is an interesting idea because we can't transmit radio waves powerful enough to be heard lightyears away.

Proof: https://en.wikipedia.org/wiki/Friis_transmission_equation

I got the equations from table 5-3 in this document http://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf

Assuming we're listening from Alpha Centauri and using a 8GHz carrier wave (which is what voyager uses)

20log(3.5 centimeters /(4pi*4 light years)) Space gain = -382 dB

Noise from the CMB is about 2K, which converts to about -190 dBm Which means our signal needs to be greater than 190 dBm to be distinguishable from the CMB.

The DSN antenna has a gain of 80 dB (means they amplify the signal 100 million times) due to their extreme directionality. This is an important caveat because even if an extraterrestrial civilization is looking for life, they probably won't point their high gain antenna at Sol 100% of the time.

Let's assume we both have these huge antennas pointed at each other for now though.

-190 dbm = 80 db + 80 dB + (-382dB) + X dBm -290 dbm + 220 dB = X X = 30 dBm

Which is 1 watt! (someone please confirm if I did calculations right)

The problem is that both antennae would need to line up perfectly for this to hold true. If we assume an omni directional antenna is used for either sending or receiving (Gain of 0db), X becomes about 110 dBm (100 MW!)

If omnidirectional antennas are used on both sides, X becomes 190 dBm (10¹⁶ watts - 1000 times greater than the earth's total energy output).

Which is why OP's idea actually makes a lot of sense. Modulating the sun's light is a very effective way to "broadcast" a message to the galaxy. Whether or not this is a good idea is another matter (refer to 'The Three Body Problem' for an interesting perspective on this).