A CFI doesn't need to be sole manipulator. He only needs to be acting as an instructor. He could do that from a back seat, IMHO. This is stretching the intent of the FAR's, I think, but I see it as loggable if it's legit.

Back seat guy is a MEI and logs it as dual given. Right seat guy is safety pilot and left seat guy is sole manipulator. All three log PIC multi. Not all three are acting as PIC, though, but that's not the point.