This is just fiction, but it's interesting nevertheless. What if ETIs stimulate the central galactic supermassive black hole to "shed" event horizons. These event horizons, hypothetically speaking, might expand superluminally to envelop the universe with very little adverse effects beyond a few million lightyears (or even less). The information might be encoded as anisotropies in the otherwise spherical expansion. Astrometrics between a few distant objects might reveal a pattern as they shift subtley out of position as the event horizons wash over them and through them.

This might prompt alot of interesting questions for and against:

Since the envelopment would affect the whole universe, would this constitute a beacon?

Other phenomena are also visible at a distance, such as supernovas. What makes a black hole better for communication?

To utilize such phenomena implies interstellar travel, maybe intergalactic travel, as a precursor. Why would such advanced ETIs need to utilize this form of communication when more conventional methods might work?