Thanks for the nice comments.  Here's some information for those 
interested in the structure of the object.

I started with my non-recursive Menger Sponger isosurface function...

function
{
  max
  (
   #declare Level = 4;
   #declare Size = 0.5;
   #while (Level > 0)
    #declare Size = Size / 3;
    min(Size-abs(mod(x,Size*6)-Size*3),Size-abs(mod(y,Size*6)-Size*3)),
    min(Size-abs(mod(y,Size*6)-Size*3),Size-abs(mod(z,Size*6)-Size*3)),
    min(Size-abs(mod(z,Size*6)-Size*3),Size-abs(mod(x,Size*6)-Size*3))
    #declare Level = Level - 1;
    #if (Level > 0) , #end
   #end
  )
}



Contained and chopped the corners off with further "max" constraints to 
form a cuboctahedron...

#declare Sponge = function
{
  max
  (
   #declare Level = 4;
   #declare Size = 0.5;
   #while (Level > 0)
    #declare Size = Size / 3;
    min(Size-abs(mod(x,Size*6)-Size*3),Size-abs(mod(y,Size*6)-Size*3)),
    min(Size-abs(mod(y,Size*6)-Size*3),Size-abs(mod(z,Size*6)-Size*3)),
    min(Size-abs(mod(z,Size*6)-Size*3),Size-abs(mod(x,Size*6)-Size*3)),
    #declare Level = Level - 1;
   #end
   x-1, -x,
   y-1, -y,
   z-1, -z,

   -x+y-z-0.5,
   +x+y-z-1.5,
   -x-y-z+0.5,
   +x-y-z-0.5,
   -x+y+z-1.5,
   +x+y+z-2.5,
   -x-y+z-0.5,
   +x-y+z-1.5
  )
}


Then made a mapping that transformed any coordinate within a sphere to a 
coordinate within the cuboctahedron such that any point on the surface 
of the sphere would also fall on the surface of the cuboctahedron...

#declare SphereTransX = function { x * sqrt(x*x+y*y+z*z) / 
max(abs(x),abs(y),abs(z),(abs(x)+abs(y)+abs(z))/2) }
#declare SphereTransY = function { y * sqrt(x*x+y*y+z*z) / 
max(abs(x),abs(y),abs(z),(abs(x)+abs(y)+abs(z))/2) }
#declare SphereTransZ = function { z * sqrt(x*x+y*y+z*z) / 
max(abs(x),abs(y),abs(z),(abs(x)+abs(y)+abs(z))/2) }


 From there I could simply define my Sphere Sponge as...

#declare SphereSponge = isosurface
{
  function
  {
   Sponge
   (
    SphereTransX(x-0.5,y-0.5,z-0.5)+0.5,
    SphereTransY(x-0.5,y-0.5,z-0.5)+0.5,
    SphereTransZ(x-0.5,y-0.5,z-0.5)+0.5
   )
  }
  contained_by { sphere { 0.5, 0.5 } }
}

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