I assume it would be done like SPATTER_ADD_. The script checks the material from the reaction and applies it, and knows which reactions to check because of the SPATTER_ADD_ in the reaction id. For creatures it would look more like SPAWN_CREATURE_, and it takes creature id and creature caste from the reaction. That way its only one script, not a new one for each reaction. SPAWN_CREATURE_CIV, SPAWN_CREATURE_PET and SPAWN_CREATURE_HOSTILE would be useful of course.
Just to let you know what I would use it for: I currently have those systems in the mod:
Golem creation and Pet Armory, both use transformations
Turrets and Landmines that are build in a factory, these are spawned. Narhirhil and deon have similar things, with robotic spiders and steam centurions.
My spawn location, which does things from sowing treant seeds (spawns 3-5 magical living trees), setting up scarecrows, or a gargoyle statue that come to life when enemies come nearby.
Deon and me also have colosseums/arenas, to summon hostile creatures to fight.
And special rocks that spawn creatures, for "random encounters" while mining.
Because if creatures could be spawned from reactions (or interactions, which would be the same in the end, I guess?) one could simulate invaders that spawn inside a fortress. I was planning on a lich that summons a skeletal army, or wormfolk that digs into your fort, shades the open portals to their realm and pour out... obviously without real diggin and real portals, all simulated, but it would work in a way.