A Cinema 4D C.O.F.F.E.E. script that creates a sky object and applies a compositing tag that has several attributes off by default and assigns a new material with all channels off except luminance.

hiddenSky.CSC

main(doc,op)
{
  // Create sky object
  CallCommand(5105);
  var sky = doc-> GetActiveObject();

  // Insert a compositing tag on selected object and adjust some attributes
  var compositingTag = AllocTag(Tcompositing);
  compositingTag#COMPOSITINGTAG_SEENBYCAMERA = 0;
  compositingTag#COMPOSITINGTAG_CASTSHADOW = 0;
  compositingTag#COMPOSITINGTAG_RECEIVESHADOW = 0;
  compositingTag#COMPOSITINGTAG_SELFSHADOW = 0;
  sky->InsertTag(compositingTag);

  // Create new material
  CallCommand(13015);

  var mat = doc->GetFirstMaterial();
  mat->SetChannelState(CHANNEL_LUMINANCE, true);
  mat->SetChannelState(CHANNEL_COLOR, false);
  mat->SetChannelState(CHANNEL_SPECULAR, false);
  mat->Update();
  mat->SetName("Sky_Material");

  // Assign material to sky object
  var skySel = doc->SetActiveObject(sky);
  CallCommand(12169);
}