Thanks. Your description makes sense, but the workflow is a little different - I assume you didn't use the Create Plot And Model View function, but just created your own viewports? I had done that before but was trying this to see what the better workflow was.
My main design layer (Design Layer 1) is where I have the set model, in which I need to see the physical instruments.
I created a "Lighting Design Layer" (Design Layer 2) to create the lighting plot (in the end I won't be doing this, I'm the set designer, but we're both working on the same 3D model.)
When I use Create Plot and Model View, that's what creates the "Definition Layer".
So it all works great now, the only small issue I see is that the LD has to use the Lighting Design Layer for all of the non-vertical positions, and the 16 different "Definition Layers" - one created for each ladder by Create Plot and Model View - for the ladders. But that's his problem :-)
Thanks so much for your help, I was pulling my hair out!