The following script might work, but if you kill the unit, say with a shot in the head. Tranquilizer dart, must have really hurt.
Anyway, here goes
Init of unit
This AddEventHandler [{Hit}, {_This Exec "Tranq.sqs"}] ; I think that s the right format at least
Script
_Unit = _This Select 0
_GunUsed = _This Select 2 ; I'm not sure if Select 2 or Select 3, or 1 is the right thing. you could test it yourself
_Tranq = "m16"
_EHHIT = _This AddEvenHandler [{hit}, {_This Exec "Tranq.sqs"}]
?_GunUsed == _Tranq
_Unit SetDammage 0
_Unit Playmove "StandSurDead"; or something like that
Exit
Looks like S*** to me, but ill make it work, when i get ther chance. This is sort of an outline.