Is this possible in a mod?

[Wolfman77]

Events can only happen once per turn at the most, for all races combined. It would work as an add on to existing WP's, but it wouldn't work very well if there were no WP's at the start of the game.

If the ancient race had a racil tech, you could give them a facility at the start that used the sector ability to cloak their planet. Just add the ability to a facility that the game setup calls for on their homeworld.

The diplomacy for an invisible AI could be chalenging though. I think you could keep them from making treaties easily, and setting them to get friendlier from other players ships and planets works pretty good, but they may still send general messages with no text if there is nothing in the speach file. Maybe a line like "Your race isn't ready for our secrets." or something like that.