You should be using create-unit, which has a -domesticate option (my little contribution to that script). I would also set the age to zero for completeness.
modtools/reaction-trigger -reactionName SUMMON_MANAWYRM -command "modtools/create-unit -race MANAWYRM -caste MALE -location \\LOCATION -age 0 -domesticate"
You may need to experiment with the location bit to make sure everything parses correctly.
Thank you Dirst!!

You're so helpful.
Hmmm, I wish I wasn't so clueless about this thing. x_x Looking in the dfhack console it looks as if
The actual reaction-trigger script is tossing errors. Looking at that line, the ooonnnnllllyyy issue I can think of is- \\LOCATION is tossing a string instead of a table?
#62
for _,arg in ipairs(command) do <-62
if arg == '\\WORKER_ID' then
table.insert(result,''..worker.id)
elseif arg == '\\TARGET_ID' then
table.insert(result,''..target.id)
elseif arg == '\\BUILDING_ID' then
table.insert(result,''..building.id)
elseif arg == '\\LOCATION' then
table.insert(result,''..job.pos.x)
table.insert(result,''..job.pos.y)
table.insert(result,''..job.pos.z)
elseif arg == '\\REACTION_NAME' then
table.insert(result,''..job.reaction_name)
elseif string.sub(arg,1,1) == '\\' then
table.insert(result,string.sub(arg,2))
else
table.insert(result,arg)
end
end
#114
if action.command then
local processed = processCommand(job, worker, worker, building, action.command) <-114
dfhack.run_command(table.unpack(processed))
end
#155
for _,action in ipairs(reactionHooks[job.reaction_name]) do
doAction(action) <-155
end
Or perhaps it wanted a string but got a table? I can't imagine \\LOCATION to give a string, I tried replacing \\LOCATION with 0 0 0 and I believe it gave the same error; I'm not sure it's even getting to that point.
Also when looking inside the scripts I noticed
-civId id
make the created unit a member of the specified civ (or none if id = -1) <-155
if id is \\LOCAL, then make it a member of the civ associated with the current fort
otherwise id must be an integer
That seems pretty awesome; although you said it works badly for citizens; any particular reason why?
Anyways I'm getting how the reaction-triggers work now, and the way it grabs other scripts- just can't actually- get it to work.
I got a feeling I'm doing something wrong that's, really freaking obvious and making myself look like an idiot.