I finally started the process of laying in the background. The most difficult and time consuming part of this was deciding on the best grey to use. Over the course of two hours I tried seven or eight different mixes and finally settled on a simple combination of Grumbacher raw umber and white: it looked more neutral than any other color and it's very easy to get an exact match if I need to make more.
Once I selected the color the actual painting part was done very quickly with thinned paint mixed with some Liquin. By alternating between a wet and dry brush I was able to let some of the background show through. I left the canvas unpainted in the bottom and bottom right corner because I liked the way it looked.
I also glazed over the shadows in the neck and head.
At this point the portrait is nearing completion so I've put together a short punch list of all the things I need to do to get it finished. The most important of these, and the next on my list, is to clean up all the edges.