That's very clever. I think it's funny that, much like while playing the game itself, accomplishing seemingly simple things while modding becomes incredibly complicated.
edit: Is it possible to restrict reactions or materials to certain entities?
Keep in mind that wood will always have a value of 3, and you cannot make it rare other than by putting it in rare biomes. I tried doing something similar with the wood and ended up just generalising it to all wood, as it just encouraged people to stock up at the embark screen/traders screen, or embarking in the relevant biome and harvesting huge amounts of wood.
Anyways, your question:
Reactions are never used by any entity. They are only relevant to the fortress currently being played, and they will be accessable to the current fortress regardless of race (unless you remove furnace operator from list of jobs). Entities will never use items made from smelter reactions in worldgen/embark.
Some materials can be restricted if you are sneaky with them. Metals more powerful than iron can be restricted by using the [METAL_PREF] tag (which starts the with race the most powerful non-deep metal IE steel atm). Metals can also be restricted from everyone by giving them the [DEEP] tag, which case they can only be produced on-site. [MINOR_METAL] restricts a race to only copper metal items, but i've heard there are some ways to exploit that.
Animals can be restricted with: [USE_EVIL_ANIMALS], [USE_GOOD_ANIMALS], [COMMON_DOMESTIC] or [USE_ANY_PET_RACE]
Items can be restricted as per the entity entries.
Most other things are only restricted by biomes. Restricting access for only one civ to certain types of stone is pretty much impossible. You could probably manage restricting trees by being sneaky with the biome crossovers, but you can still end up with civs that straddle both biomes and get the trees anyway.