How can I customize the way the POVRay image is rendered?

You can edit the file tf_land.pov to your liking as long as you keep the TF* placeholders for terraform to substitute. Alternativeley you can also point terraform at another POV file through the Options dialog or the terraform resource file. Whatever POV file terraform uses must contain the TF* placeholders for terraform to be able to use it. If you come up with something neat, consider sending me the POV file, so I can add it to the terraform distribution.