That could be made to work, but you'd still need to tune the factors. What if the "lit" part wasn't very bright? That would look like a shadow too.

I think I found an easier way. Poser has a strange quirk - IBL cannot create a specular effect.

So - don't use a Diffuse node, use a Specular node. It can only detect directional light. It cannot detect IBL. That's half the solution. If only the Specular node would behave like the Diffuse node and produce a broad reflection.

But wait - if you set the roughness very large, it does!

In this demo setup, I'm not really lighting the ball. I'm coloring it red or blue - red is in shadow, blue is lit by directional light. (An infinite in this case.) The red part is connected to an AO node. The blue part is not and will have no AO applied to it.

Of course, you should not use red and blue - just use white, and connect that to your Diffuse_Value.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)