So, after lots of trial and error, I've decided upon an approach for the fly-through sequence of Chasm. Shadowed lights in volumes were just taking too long in the renderer. So instead (with Ethan's help) I've opted to use attenuated lights to help shape these clouds. This is my first pass at this approach. There's a lot left on it, but I thought I would share it. I'm once again hopeful that I can pull this off.