First of all, your post is so long that I only skimmed over it, so I 
apologize if I misunderstood something. But I still think I have to 
comment on some points.

> older Macs are standardized around 1.8

Not just older Macs. I've never seen it documented anywhere, but 
according to what Apple's display calibration utility does, that number 
is still valid.

> We all assume --don't we?--that POV's
> color/brightness values, as used in a typical PIGMENT block, are meant to
> reproduce brightness levels such that <.5,.5,.5> represents "half as
> perceptually bright as" <1,1,1>.

No. At least I don't assume that, and I think most other people with 
some background in physically-based color theory and computer graphics 
don't either.

For me, from the point of view of the POV-Ray scene, <.5, .5, .5> means 
"that surface's diffuse reflectivity is half that of the other one" with 
  <1, 1, 1>. From the point of view of the rendered image displayed on a 
computer screen, it means "the physically measurable light intensity 
(W/m^2) emanating from these pixels is half the one emitted by those 
pixels".

Of course that means that you and I are starting from different 
premises, so there's probably no point in any further discussion about 
the fact that I think assumed_gamma 1.0 is correct. That's why I allowed 
myself to answer before having thoroughly read your whole argument.

  -Christian

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