Sending a signal at interstellar distance requires enormous power - something of order not too far from energies of the stars. Even if we harness the power of stars, modulating that signal would take a lot of energy - though the actual power required could be reduced - at cost of bandwidth. Imagine a swarm of statites near Oort Cloud, designed for such task: crafts with solar sails large enough to eclipse the Sun as seen from other star to a visible degree, able to turn the sails to reveal it or obscure it - modulate Sun brightness as seen from the distant destination - but the process of turning the statite takes a week. So our bandwidth is of order of 1 bit per week.
Does SETI search signals modulated as slowly? Transmissions of order of bit per week, month, year?