I think likeness to a real skull can be good but it doesn't have to be organic. That would make it feel like a spray painted human skull. I think it would be more effective to paraphrase the organic stuff into mechanical components of the same relation and proportion as a skull. Kinda like those human faces made from vegetables. You find a suitable fruit. Robotic shapes are... carved and simple, aligned, flat surfaces, curves, circles... in general, easy to compress into a 3D description (in a CAD program for example).
http://www.amazon.com/Robotic-Human-Silvered-Statue-Android/dp/B000RQ389M http://www.cyberpunkreview.com/movie/decade/1990-1999/hardware/If you look on the sides of the nose on a human skull, you'll see 2 holes. I'm guessing blood veins go through there. Anyways, those holes could be made into... something geometrically simple and robotic. Vents, rivets, screws. Just an example. Edit: there are also holes in the 'eyebrow' bone and jaw.
