I found the answer how the opacity level for object3d can be controlled: One has to tweak Config.glTransparencyOffset and Config.glTransparencyMul (for opengl renderer).
Once again, thanks for the great support!

Egon, thanks for your quick reply!
I just thought of a better way to achieve the same effect as described in my previous post. If the world is rendered twice into same framebuffer, whereby the depth buffer is cleared in between, it should be the same (If the world is set transparent the second time). In a first step I tried without transparency. The below code snippet shows this, the world is drawn the second time a bit rotated to see the effect. I would assume that the world drawn in red (because of glColorMask) would win in the Z-Buffer, because it is drawn after the world drawn in white, however it gets blended over the white rectangle (As can be seen in the image).

      buffer.clear();     

      GL11.glColorMask(true,true,true,false);
     
      world.renderScene(buffer);
      world.draw(buffer);
      buffer.update();
           
      buffer.clearZBufferOnly();
 
      camera.rotateY(-rot);
     
      GL11.glColorMask(true,false,false,false);
     
      world.renderScene(buffer);
      world.draw(buffer);
      buffer.update();
           
      camera.rotateY(rot);
     
      GL11.glColorMask(true,true,true,false);
      buffer.displayGLOnly();

The image:
[img=http://img523.imageshack.us/img523/7402/effectmr2.th.jpg]

Thanks for any suggestions!