Tammar,
  I would create two bitmaps with MRIcroGL, using the clip planes and shaders so that one image shows the far hemisphere as opaque and hides the near hemisphere, while the other shows the near hemisphere with the nearby activations shining through. You can then use your favorite bitmap editor to blend the two together to get the desired effect. There is even an online tool for this:
  http://tma.main.jp/blend/