Wabe is right, but in many cases you can bypass this limitation:  if you rotate the object with one of its surfaces facing the sky, then you should be able to populate this surface. What you must do:

• give this surface a unique material id.
• uncheck "decay near foreign object" for the material.
• rotate the object to make the surface face the zenith
  -populate the individual material of the surface
  -rotate back the object.

(image of a simple sphere with four different ecomats)