Well,
Seen some interesting things, and maybe a combo which is much like you asked for.
If you want to maintain teammate status but deny combat, it is possible to use setghost, + drop the confidence setting of the follower down to zero. The follower will end up doing cower idles and/or running away instead of fighting, and it does not seem that mobs will aggro on it. The assistance and aggression settings don't seem to make a difference in this case. You get a flee behavior which can include cower idles and associated comments (which can make the thing inappropriate for use on humans, depending on what you want). But on like, Rex, you don't recognize any cower idles for what they are. Flee behavior does not look so cowardly on an animal follower.
What I am seeing with setghost alone is, it does a very good job of preventing mobs from aggroing on the follower. You can take the follower and dump it directly onto any aggressive mob via moveto and as long as the player's own combat state is not involved, nobody cares, and the follower will just run back. However, if 'player teammate' status is live, followers will combat/aggro bigtime on anything which is seeking active combat with the player (IE: you see 'Caution' or worse over your head when you are in sneakstate). It is almost the passive mode I was hoping to make myself, except that the follower does not wait for the player to attack, the follower is doing it based upon the player *being* attacked. This is a key difference.
As far as creating complete pacificity out of a follower with no visual acknowledgement of player combat whatsoever (which btw is what I myself am trying to make as a starting point). One way is to remove teammate status, and then one can lower its assistance to 0, plus use setghost. I know of another way which involves using an ambush package - - which happens to work beautifully in Fallout 3 - - but it's got some freaky caviats which don't seem to obey line of sight and can make very strange things happen now and then. Specifically. When an ambush package has a really small radius to it, the follower seems actually unaware that the player is being attacked if everything is outside of that radius. But if that radius has any size to it to speak of, if something enters the radius, the follower seems to have instant, magical knowledge of where it is and will path halfway across the earth to reach it (for example, maybe the mob was on the other side of a wall). Makes things interesting when that happens.
It is also possible to replace the removal of teammate status + assistance change with using setrestrained on the follower, but you sorta have to do it while the right animation is happening or you have the follower doing an animation and sorta unable to stop (it also goes without saying that it ruins the ability to 'follow'). Kinda not so useful to most follower-related situations I think.
Edit: If the ghost teammate aggro situation happened upon the "DANGER" detected status on the player and not the "CAUTION" one, it would be close enough to the text description I saw in that little brown book to fly as a straightforward passive mode. That would be like a dream come true for follower behavior, actually.
this made me think. can you have child companions? because children cant/dont attack afaik. maybe if you could do to your companions whatever it is that makes children unkillable?
That's true! I've never even examined what they did with children tbh (I don't know if NV put it in, even, never thought to shoot at the kids I saw). I know that I have seen kids cower, though. What I saw out of the kids reminds me actually of the setghost+confidence thing I saw just now. I'll have to check next time I see a kid in game (I don't know if NV made them unkillable but I wouldn't be surprised).