This has been suggested hundreds of thousands of times over the years. This exact suggestion.

It wouldn't necessarily be useful though and here is why. When you spawn an S rank, for example by killing all 300 enemies in Mare Lamentorum, you have to find and locate the S rank. This means you fly around the area, look for it and travel to it. This can take a few minutes. Then you relay the information into linkshells. It takes about 2 minutes for people to arrive. By then, the 5 minutes of invulnerability is up.

Some spawners would deliberately spawn it and wait the 5 minutes before relaying, so that the invulnerability timer expires before people start server jumping, making it irrelevant.

There is really no reason for it on Forgiven Rebellion or Ker. The minion spawns act as advance notice it will spawn.