Re: I guess you have measured the intensity at points on the skin of the body...
AFAIK = as far as I know
FWIW = for what its worth
As far as scalp zapping goes, I can not share a lot of expertise.
One way that I might consider is:
Wash hair, rinse thoroughly, soak in salt water, wrap and cover with a paper towel and then place the positive electrodes at the top of the scalp. I would then close my eyes and turn the unit on. If I suddenly saw light at any significant brightness, I would turn it back off. The light might be an indication of electricity passing through the eyes or down the optic nerve.
I would also do this in a very quiet place where I could hear a pin drop. When the zapper was turned on, I would listen for any whine,whistle, squeal, or other noise that appeared when the zapper was on and that went away when the zapper was off.
If everything appeard to be safe, I would continuing following a normal zapping routine.
This is just the way that I would try it.