Back to Rush:
I think I'd rate both A Farewell to Kings and Hemispheres ever-so-slightly ahead of 2112. My two cents.
As for the naked guy, he's a contrast to the banker-like figure. Like the two hemispheres of the brain, the two figures represent opposite ways of thinking & being. The banker is rational, uniform, conventional. The dancer is artistic, unselfconscious, natural.
The naked figure on Hemispheres also hearkens back to the Vitruvian Man-like imagery on the cover of All the World's a Stage. It's another image about duality, contrasting the uniformity of the star and circle shapes with the naked human figure, who appears to be trying to escape from -- or at least resist -- their controlling confines.
Or at least that's how I think about it!