floor/ground which masks illumination from the bottom hemisphere anyway... Even if the horizon was dragged down only about 10-15 degrees, before fading to black/grey, it would help 90% of the cases where you see a slight bit below the horizon just because your scene doesn't extend to infinity.
I'll do some experiments with fading to grey. (or maybe the 'average' sky colour, .i.e. the colour a grey surface would be if lit by the sky)
Would be cool to eventually see your method for generating sky data as a plugin or standalone app with some of those old cloud tests and a spherical camera and things to generate different sky maps :) Could be a neat toy to simplify making cool custom skies for your Indigo (and other) renders!
EDIT: Then again, I suppose that's what programs like Terragen are there for.