I wonder if it would be better to just use a ground/scene props then to capture the shadows, and the dome on top with a sky planted on it, rather than what was mentioned on bagginbills site. I curious though how he got the shadows to appear on the last few renders here:

https://sites.google.com/site/bagginsbill/free-stuff/environment-sphere/environment-sphere-demo-renders

Looks like it was possibly done by rendering shadows separately then using photoshop.