What happens if you make something alpha enabled?
Then skinners can make parts of the skin invisible.
Yes, you can apply an "alpha channel" to that part of the mesh.
Think of the alpha channel as a black and white image. Any area that is black on the channel will not show up in the final rendering of the mesh. Anything that is white, will be visible. So if you want a man with one hand completely invisible, you can do it by applying an alpha channel that is all white except for the hand, which you'd color black.
You can also take advantage of the range of grays from black to white, to produce objects that range in transparency... or by applying a color (in nifskope) various effects can be achieved, like making meshes look like glass, water, ice, crystal, ghostly, etc.
For Iceman, I've been playing around with different settings to see what happens. Making him semi-transparent looks nice, just like the modern version, who turns completely to ice. But when I put the particle mist effect on, it sometimes causes the thing they cover to go completely invisible.
Either there is a limit to the number of layers you can alpha, or there is a setting somewhere that I'm missing...