Yu could just go into the Material Room; set one part of a figure's Diffuse Color to white(Without any Textures,) then that parts Transparency to about 80-90%, and then use the Apply to All feature, near where you create a new node from, in the top right of the Node Palette.  You'll probably need to set internal parts, like Teeth, Tongue etc to %100 transparent.

An other option is to just render as usual, and then alter the Opacity of your figure in a 2D Image Editor, like Photoshop.