ZDoom

Neat! Thank you very much.

Here's hoping it gets added as an "official" feature, but this will certainly work for me.

Class ActivatedDecal : Decal{
	Default{+DORMANT; +NOGRAVITY;}
	Override void BeginPlay(){}
	Override void Activate(Actor activator){
		SpawnDecal();
		Destroy();
	}
}

Until someone either adds it or closes this thread, I just wanted to say that you can make it work using ZScript:
[code=php]
Class ActivatedDecal : Decal{
Default{+DORMANT; +NOGRAVITY;}
Override void BeginPlay(){}
Override void Activate(Actor activator){
SpawnDecal();
Destroy();
}
}
[/code]
All this does is move the functions from BeginPlay to Activate to make it respond to Thing_Activate in ACS.

Could activatable decals be added?

At present, the [url=https://zdoom.org/wiki/Classes:Decal]decal thing[/url] can be use to place a decal in a map. However, the decal always appears at map initialisation. Even if the decal thing is set to dormant, the decal will spawn.

It would be useful if it was possible to place a decal thing in a map but not have it actually generate a decal until the thing is activated via ACS.

That way, you could make changes to the appearance of an area by spawning bullet holes/rocket scorches, putting posters on the walls, graffiti, whatever, to indicate some change in the area from the last time the player was there.