The best way to achieve my initial question in p.advanced-users is the 
solution given by Alain: using a torus function for the density:

#local Radius = 30;
#local F_pig = pigment {function {f_torus(x,y,z, 1.0, 0.15)}};

torus {1.0, 0.15
   hollow
   material {
     texture {
       pigment {rgbt 1}
     }
     interior {
       media {
         intervals 1
         samples 10
         emission 0.5/Radius
         density {
           pigment_pattern {F_pig}
           color_map {
             [0.8 rgb <0,1,0>]
             [1.0 rgb <1,0,0>]
           }
         }
       }
     }
   }
   scale Radius
   rotate 10*x
   rotate 15*z
}


Now, I need to investigate further in combining this with toroidal warps 
and other media matter. My ultimate goal is to achieve a credible Milky 
Way that can be seen from a planet for instance.

Thomas

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