If you don’t want to hurt the darlings, what is wrong with giving them the same stuff that knocks people out before surgery but just give them a whole lot of it (or something else) when they are out? I was looking around and found a list that mentioned propofol, etomidate, methohexital, pentothal, and ketamine. I don’t know anything about these, but I would imagine they would work.
Nothing, if as soon as they are "out" you remove all useful body parts, drain all blood and dispose of the rest. Recycling, you know.