projectileExpansion. It's all about the raws here. It only runs when projectiles hit and there shouldn't be *too* much of a performance hit.

projectileExpansion.lua

-- Adds extra functionality to projectiles. Use the argument "disable" (minus quotes) to disable.

flowtypes = {
miasma = 0,
mist = 1,
mist2 = 2,
dust = 3,
lavaMist = 4,
smoke = 5,
dragonFire = 6,
fireBreath = 7,
web = 8,
undirectedGas = 9,
undirectedVapor = 10,
oceanWave = 11,
seaFoam = 12
}

local function posIsEqual(pos1,pos2)
    if pos1.x ~= pos2.x or pos1.y ~= pos2.y or pos1.z ~= pos2.z then return false end
    return true
end

local function getMaterial(item)
    if not dfhack.matinfo.decode(item) then return nil end
    return dfhack.matinfo.decode(item).material
end

local function getSyndrome(material)
    if material==nil then return nil end
    if #material.syndrome>0 then return material.syndrome[0]
    else return nil end
end

local function removeItem(item)
    item.flags.garbage_collect = true
end

local function findInorganicWithName(matString)
    for inorganicID,material in ipairs(df.global.world.raws.inorganics) do
        if material.id == matString then return inorganicID end
    end
    return nil
end

local function getScriptFromMaterial(material)
    local commandStart
    local commandEnd
    local reactionClasses = material.reaction_class
    for classNumber,reactionClass in ipairs(reactionClasses) do
        if reactionClass.value == "\\COMMAND" then commandStart = classNumber end
        if reactionClass.value == "\\ENDCOMMAND" then commandEnd = classNumber break end
    end
    local script = {}
    if commandStart and commandEnd then
        for i = commandStart+1, commandEnd-1, 1 do
            table.insert(script,reactionClasses[i].value)
        end
    end
    return script
end

local function getUnitHitByProjectile(projectile)
    for uid,unit in ipairs(df.global.world.units.active) do
        if posIsEqual(unit.pos,projectile.cur_pos) then return uid,unit end
    end
    return nil
end

local function matCausesSyndrome(material)
    for _,reactionClass in ipairs(material.reaction_class) do
        if reactionClass.value == "DFHACK_CAUSES_SYNDROME" then return true end --the syndrome is the syndrome local to the projectile material
    end
    return false
end

local function alreadyHasSyndrome(unit,syn_id)
    for _,syndrome in ipairs(unit.syndromes.active) do
        if syndrome.type == syn_id then return true end
    end
    return false
end

local function assignSyndrome(target,syn_id) --taken straight from here, but edited so I can understand it better: https://gist.github.com/warmist/4061959/
    if target==nil then
        return nil
    end
    if alreadyHasSyndrome(target,syn_id) then
        local syndrome
        for k,v in ipairs(target.syndromes.active) do
            if v.type == syn_id then syndrome = v end
        end
        if not syndrome then return nil end
        syndrome.ticks=1
        return true
    end
    local newSyndrome=df.unit_syndrome:new()
    local target_syndrome=df.syndrome.find(syn_id)
    newSyndrome.type=target_syndrome.id
    --newSyndrome.year=
    --newSyndrome.year_time=
    newSyndrome.ticks=1
    newSyndrome.unk1=1
    for k,v in ipairs(target_syndrome.ce) do
        local sympt=df.unit_syndrome.T_symptoms:new()
        sympt.ticks=1
        sympt.flags=2
        newSyndrome.symptoms:insert("#",sympt)
    end
    target.syndromes.active:insert("#",newSyndrome)
    return true
end

function getProjectileExpansionFlags(material)
    local projectileExpansionFlags = {
    matWantsSpecificInorganic = 0,
    matWantsSpecificSize      = 50000,
    matCausesDragonFire       = false,
    matCausesMiasma           = false,
    matCausesMist             = false,
    matCausesMist2            = false,
    matCausesDust             = false,
    matCausesLavaMist         = false,
    matCausesSmoke            = false,
    matCausesFireBreath       = false,
    matCausesWeb              = false,
    matCausesUndirectedGas    = false,
    matCausesUndirectedVapor  = false,
    matCausesOceanWave        = false,
    matCausesSeaFoam          = false,
    matHasScriptAttached      = false,
    matCausesSyndrome         = false,
    returnLocation            = false,
    matDisappearsOnHit        = false
    }
    local matName  = nil
    for k,reactionClass in ipairs(material.reaction_class) do
        if debugProjExp then print("checking reaction class #" .. k .. "...",reactionClass.value) end
        if string.find(reactionClass.value,"DFHACK") then 
            if debugProjExp then print("DFHack reaction class found!") end
            if reactionClass.value == "DFHACK_SPECIFIC_MAT"     then matName                                           = material.reaction_class[k+1].value           end
            if reactionClass.value == "DFHACK_FLOW_SIZE"        then projectileExpansionFlags.matWantsSpecificSize     = tonumber(material.reaction_class[k+1].value) end
            if reactionClass.value == "DFHACK_CAUSES_SYNDROME"  then projectileExpansionFlags.matCausesSyndrome        = true            end        
            if reactionClass.value == "DFHACK_DRAGONFIRE"       then projectileExpansionFlags.matCausesDragonFire      = true            end
            if reactionClass.value == "DFHACK_MIASMA"           then projectileExpansionFlags.matCausesMiasma          = true            end
            if reactionClass.value == "DFHACK_MIST"             then projectileExpansionFlags.matCausesMist            = true            end
            if reactionClass.value == "DFHACK_MIST2"            then projectileExpansionFlags.matCausesMist2           = true            end
            if reactionClass.value == "DFHACK_DUST"             then projectileExpansionFlags.matCausesDust            = true            end
            if reactionClass.value == "DFHACK_LAVAMIST"         then projectileExpansionFlags.matCausesLavaMist        = true            end
            if reactionClass.value == "DFHACK_SMOKE"            then projectileExpansionFlags.matCausesSmoke           = true            end
            if reactionClass.value == "DFHACK_FIREBREATH"       then projectileExpansionFlags.matCausesFireBreath      = true            end
            if reactionClass.value == "DFHACK_WEB"              then projectileExpansionFlags.matCausesWeb             = true            end
            if reactionClass.value == "DFHACK_GAS_UNDIRECTED"   then projectileExpansionFlags.matCausesUndirectedGas   = true            end
            if reactionClass.value == "DFHACK_VAPOR_UNDIRECTED" then projectileExpansionFlags.matCausesUndirectedVapor = true            end
            if reactionClass.value == "DFHACK_OCEAN_WAVE"       then projectileExpansionFlags.matCausesOceanWave       = true            end
            if reactionClass.value == "DFHACK_SEA_FOAM"         then projectileExpansionFlags.matCausesSeaFoam         = true            end
            if reactionClass.value == "DFHACK_DISAPPEARS"       then projectileExpansionFlags.matDisappearsOnHit       = true            end
        end
        if reactionClass.value == "\\COMMAND"                   then projectileExpansionFlags.matHasScriptAttached     = true            end
    end
    if matName then projectileExpansionFlags.matWantsSpecificInorganic = findInorganicWithName(matName) end
    return projectileExpansionFlags
end

debugProjExp=false

events=require "plugins.eventful"
events.onProjItemCheckImpact.expansion=function(projectile)
    if debugProjExp then print("Thwack! Projectile item hit. Running projectileExpansion.") end
    if projectile then
        if debugProjExp then print("Found the item. Working on it.") end
        local material = getMaterial(projectile.item)
        if not material then return nil end
        local projectileExpansionFlags=getProjectileExpansionFlags(material)
        if debugProjExp then print(projectileExpansionFlags) printall(projectileExpansionFlags) end
        local syndrome = getSyndrome(material)
        local emissionMat = projectileExpansionFlags.matWantsSpecificInorganic --defaults to iron
        local flowSize = projectileExpansionFlags.matWantsSpecificSize         --defaults to 50000
        if projectileExpansionFlags.matCausesDragonFire      then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.dragonFire,0,0,flowSize) end
        if projectileExpansionFlags.matCausesMiasma          then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.miasma,0,0,flowSize) end
        if projectileExpansionFlags.matCausesMist            then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.mist,0,0,flowSize) end
        if projectileExpansionFlags.matCausesMist2           then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.mist2,0,0,flowSize) end
        if projectileExpansionFlags.matCausesDust            then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.dust,0,emissionMat,flowSize) end
        if projectileExpansionFlags.matCausesLavaMist        then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.lavaMist,0,emissionMat,flowSize) end
        if projectileExpansionFlags.matCausesSmoke           then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.smoke,0,0,flowSize) end
        if projectileExpansionFlags.matCausesFireBreath      then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.fireBreath,0,0,flowSize) end
        if projectileExpansionFlags.matCausesWeb             then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.web,0,emissionMat,flowSize) end
        if projectileExpansionFlags.matCausesUndirectedGas   then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.undirectedGas,0,emissionMat,flowSize) end
        if projectileExpansionFlags.matCausesUndirectedVapor then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.undirectedVapor,0,emissionMat,flowSize) end
        if projectileExpansionFlags.matCausesOceanWave       then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.oceanWave,0,0,flowSize) end
        if projectileExpansionFlags.matCausesSeaFoam         then dfhack.maps.spawnFlow(projectile.cur_pos,flowtypes.seaFoam,0,0,flowSize) end
        if projectileExpansionFlags.matHasScriptAttached or projectileExpansionFlags.matCausesSyndrome then
            local uid,unit = getUnitHitByProjectile(projectile)        
            if projectileExpansionFlags.matHasScriptAttached then
                local script = getScriptFromMaterial(material)
                for k,v in ipairs(script) do
                    if script[k] == "\\UNIT_HIT_ID" then script[k] = unit.id end
                    if script[k] == "\\LOCATION" then
                        script[k] = projectile.cur_pos.x
                        table.insert(script,projectile.cur_pos.y,k+1)
                        table.insert(script,projectile.cur_pos.z,k+2)
                    end
                end
                    dfhack.run_script(table.unpack(script))
                end
            if projectileExpansionFlags.matCausesSyndrome then assignSyndrome(unit,syndrome.id) end
        end
--      if projectileExpansionFlags.matDisappearsOnHit then dfhack.items.remove(projectile) end
    end
    return true
end

if ... ~= "disable" then 
    print("Enabled projectileExpansion.")
else
    events.onProjItemCheckImpact.expansion = nil
    print("Disabled projectileExpansion.")
end