First I set to "place sign which looks like ID = x". But on maps with too many events this crashed the game as it seems there is a limit to sign texts.
So then I just changed the graphics of mithril mine to event graphic and now I just !!UN:Iv1/v2/v3/63/27;
It will place this event graphics and surprisingly, probably because there is an event under, the square remains passable and no yellow trigger.