It should be do-able in Bryce with a good greyscale image, but you would only get the face without the back of the head. You could export the terrain as an obj file and then import into a modeller application like Wings3D or Anim8or and reshape and model the back and maybe some of the side of the head... If you add the back and side of the head in Bryce with boolean or metaballs or such, I doubt that it then can be exported into obj - but terrains can be exported into a useful format. If you just need the face part, it would be worth a try with a greyscale highfield map in the terrain editor of Bryce or maybe even in Terragen.