Is a certificated pilot in a rear seat with no access to flight controls a safety pilot or a passenger?
After looking through the FAR's, the answer is no, the rear passenger seat cannot be occupied by somebody claiming to be a "safety pilot".

FAR 91.109(c):


  (c) No person may operate a civil aircraft in simulated instrument flight unless—
  
  &nbsp;&nbsp;(1) The other control seat is occupied by a safety pilot who possesses at least a private pilot certificate with category and class ratings appropriate to the aircraft being flown.
  &nbsp;&nbsp;(2) The safety pilot has adequate vision forward and to each side of the aircraft, or a competent observer in the aircraft adequately supplements the vision of the safety pilot; and


(Emphasis Mine)

So the safety pilot basically has to be able to access the controls and have vision that supplements the pilot, meaning must be able to see as well as the pilot position would.