Thinking out loud, we really need a way to temperature-texture prints. For example, instead of applying a colour-map texture as you would for a full-colour print in Shapeways sandstone, use the colour-map texture to “pull” the nozzle temperature hotter/colder. If the temperature swings happen quickly enough, it should result in visibly darker/lighter patches in the wood print.
http://blog.ultimaker.com/2013/08/09/water-lily-by-dizingof-in-wood-pla/