I built up the image in the background, much the same way I did the figure, by slowly building up layers of "paint." At first I tried to use the same brush that had worked so well on the figure, but because it was a custom brush, Photoshop kept getting bogged down with rendering it. Instead I switched to a round preset brush. I set it to almost 0% hardness so the brush strokes would look soft. The blurry edges helped me get a feel for the values without getting caught up in details I would probably eventually paint over anyway. It also aided with atmospheric perspective.
I am going to hazard a colored version, although I've been quite satisfied with the grisaille at this point.