Some good stuff on there.
One thing it is difficult to find is a large tower case specifically designed for passive cooling. Most cases are just generic fan cases with the fans removed.
I would have thought a case designed on the same principle as a solar chimney could work well.
Solar chimney - Wikipedia, the free encyclopedia
That's the idea, but no need to use the sun, the components provide the heat, and an underground thermal exchange would be a bit unrealistic.
So, essentially a case which is quite similar to a small wood burning stove.
It could draw air in from the bottom, and have some kind of short chimney on the top.
Using the chimney effect to get air moving would help keep things like the motherboard cooled, and stop pockets of warm air being trapped in corners etc.