I've found what I call the "door mat" approach to work nicely. Take an object with a flat top and recess it into the floor until it sticks just above the surface, like a doormat. :p
Obviously, after you recess the object, you place the NPC on top of it. Hmm... you could probably use the same approach for an NPC that was laying on a regular prop too. Such as an unconscious man laying on a bench.
I got the invisible object trick to work. I added two of them, one to appear after I complete a different objective to perform a different action that is suppose to beam them out. I tried to add a duplicate of the same officer that appears after I complete the second component, and then beam out. Problem is he beams in. I don't really want that to happen. How would I get him to beamout and stay beamed out? Or would this be another invisible object trick?
Nah. Didn't work. Just disappeared too. Disappearing is just gonna have to do it. I guess it is one of the many limitations that the Foundry creators didn't work out. But who knows. Maybe it'll be different when it goes live.