Alright, so I figured out how to make the reaction work.
First, I created a new material, one that should only be produced when Adamantine wafers are smelted. It's basically the same name, because I really didn't care what to call it, save for the gas since that's the object I was trying to produce in the first place.
[INORGANIC:PROTO_ADAMANTINE]
[ITEM_SYMBOL:157]
[STATE_NAME_ADJ:ALL_SOLID:raw adamantine]
[STATE_NAME_ADJ:LIQUID:molten raw adamantine]
[STATE_NAME_ADJ:GAS:raw adamantine vapor]
[DISPLAY_COLOR:3:0:1]
[TILE:156]
[MATERIAL_VALUE:250]
[SPEC_HEAT:7500]
[MELTING_POINT:NONE]
[BOILING_POINT:10000]
[MAT_FIXED_TEMP:10001]
[SOLID_DENSITY:200]
[LIQUID_DENSITY:2600]
[MOLAR_MASS:55845]
[IMPACT_YIELD:2500000]
[IMPACT_FRACTURE:3000000]
[IMPACT_STRAIN_AT_YIELD:0]
[COMPRESSIVE_YIELD:2500000]
[COMPRESSIVE_FRACTURE:3000000]
[COMPRESSIVE_STRAIN_AT_YIELD:0]
[TENSILE_YIELD:2500000]
[TENSILE_FRACTURE:3000000]
[TENSILE_STRAIN_AT_YIELD:0]
[TORSION_YIELD:2500000]
[TORSION_FRACTURE:3000000]
[TORSION_STRAIN_AT_YIELD:0]
[SHEAR_YIELD:2500000]
[SHEAR_FRACTURE:3000000]
[SHEAR_STRAIN_AT_YIELD:0]
[BENDING_YIELD:2500000]
[BENDING_FRACTURE:3000000]
[BENDING_STRAIN_AT_YIELD:0]
[MAX_EDGE:1000] no swords until you can pick mats
[ITEMS_HARD]
[IS_STONE]
[SYNDROME]
[SYN_NAME:Adamantine Mutation]
[SYN_AFFECTED_CLASS:GENERAL_POISON]
[SYN_CONTACT][SYN_INHALED][SYN_INGESTED][SYN_INJECTED]
[CE_ADD_TAG:EXTRAVISION:NO_PHYS_ATT_RUST:NO_FEVERS:NONAUSEA:NO_DIZZINESS:NOFEAR:PARALYZEIMMUNE:SEV:100:PROB:100:START:50:PEAK:100]
[CE_PHYS_ATT_CHANGE:STRENGTH:200:0:AGILITY:200:0:TOUGHNESS:400:0:ENDURANCE:400:0:RECUPERATION:600:0:DISEASE_RESISTANCE:800:0:START:50:PEAK:100]
[CE_MENT_ATT_CHANGE:ANALYTICAL_ABILITY:200:0:FOCUS:200:0:WILLPOWER:400:0:INTUITION:600:0:SPATIAL_SENSE:600:0:KINESTHETIC_SENSE:600:0:START:50:PEAK:100]
[REACTION:ADAMANTINE_WAFERS]
[NAME:make adamantine wafers]
[BUILDING:SMELTER:NONE]
[REAGENT:A:15000:THREAD:NO_SUBTYPE:METAL:ADAMANTINE]
[PRODUCT:100:1:BAR:NO_SUBTYPE:METAL:ADAMANTINE][PRODUCT_DIMENSION:150]
[PRODUCT:100:1:BOULDER:NO_SUBTYPE:INORGANIC:PROTO_ADAMANTINE][PRODUCT_DIMENSION:150]
[FUEL]
[SKILL:SMELT]
So when one makes adamantine, hopefully small clouds of the stuff appear, because the metal has very low boiling point. This means I'd have my military dwarves on smelting duty for wafers, and if I want the syndrome to affect the plant life then I'll have to put the smelter outside.
So I decided the best way to test this would be in the arena, using the createitem script from dfhack.
First, I placed the "boulders" all around a dwarf.
Apparently I may need to set a lower boiling point, or it may just be that immediately spawning them in doesn't melt them immediately. Either way, I had to crank up the temperature from warm to hot. I got what I wanted, obviously. Feels like I should rename the vapor itself, actually.
And it works.
Like...really works.
Barely even a bruised leg.
So I suppose I'll have to fix it to make the increases more gradual, up to a point, as well as seeing if I can get the gas to affect the plants so that any animals I have pastured will also be affected.
Which, by the way, would include any type of animal...