Perhaps some physicist can help me out, here, but I don't know if the phenomenon you describe, even if it was possible, would also be superluminal. At any rate, any species capable of tickling black holes and making them shed event horizons, FTL or not, would certainly have the ability to generate supernovae at will. Since SNs are typically as bright as the galaxies they occur in, they would be visible at colossal distances, as far as the galaxies themselves. If one of these civilizations chose to use SNs to announce their presence to the surrounding universe, how would this look to an external observer?

Unless these artificial SNs distinguished themselves somehow, perhaps by unusual spectra or a modulated light curve, they would be indistinguishable from ordinary SNs. At first, I thought you might be able to use them as signalling devices, perhaps by setting them off in groups, geometrical patterns, etc. But galaxies are tens of thousands of light years in size, and setting off several supernovae "simultaneously"
would still mean the flashes would arive at an external observer millenia apart. If you selected a group of stars very close to one another, say in a cluster, and detonated them, it might be mistaken by an external observer as some form of natural chain reaction effect, not an artifact.

The fact that we do not see Type III Civ events of this type suggests that extremely advanced civilizations do not exist, we can see SNs as far away as we can see galaxies. Another possibility is that civilizations this advanced may not want to meet any neighbors comparable to them in power.