I'm wondering if a math function could take care of that. I was also thinking with a transparency map gradient oriented for density distance (along the z axis) or height (along the y axis) could be figured into the node tree. Now this is complex for me to figure out, but as an abstract idea to hand to a guru such as bagginsbill perhaps some interesting effects could be developed.

Here is a rough render to show what you guys have helped me with so far ...

Boni

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"Be Hero to Yourself" -- Peter Tork