There should be a client sided mod, activated by double tapping the freelook key, where when you stand still for about a second it automatically enables freelook, even in first person. When any movement keys are pressed, the camera is normal again.
(Optional: when you move, the avatar rotates to face forward where you were looking, looking too far brings your body to face 180° around) other people will see your head turn.
Essentially the camera system in Destiny
Yes? No? Impossible?
EDIT
This wouldn't happen when you're holding an item, so any DM won't have headturn.
