it turns out that it's not necessary to use IDL IC=100.  using the simple glass refractive/reflective
shader with approx. 0.01 diffuse, I'm seeing the hdri lite go into the glass-front box and it
appears to be bouncing around.  a lite meter inside the box appears to detect about 50% of
the illumination seen by a lite meter outside the box, which might be expected in reality
(darker inside than outside during the day).  I can't tell if the glass is cutting down on diffuse rays
from the hdri as they enter the box.  will try to post render later.