Hi, being interested in experimental map modelling that could possibly run on Unvanquished or Xonotic using Darkradiant, GIMP and Blender, I did some research in order to find a software that would be able to do relief generation and I found out Geomorph that might interest you.
I'm myself experimenting to make a lunar land for the Iron Sky Nazi Lunar base (the one from the trailer, not the one from the movie that is different) and a lunar castle (I have Wolfenstein in mind).
As an amateur 3D modeler, I recommend this software (Geomorph), which is the only one (with Fracplanet to a lesser extent) to be able to generate various types of procedural terrain.
Thank you, Madam, Sir, for your attention to this letter.
Badly I just forgot this e-mail until it came to my mind again while reading Ingar talking about Terragen on quetoo's IRC channel.
Did you know that software? have you ever experimented with this? The same way Ingar talked about Terragen for skybox generation purpose, Geomorph can probably be used on that purpose. Geomorph is a free software that relies on POV-Ray for rendering, I just discovered that POV-Ray is now open-source and free too.
For terrain generation, there is also that other software named TremGen written by some friends from when I was a student! This one is able to produce .map files ready to be loaded by NetRadiant.