I have left the Grimoire marinating in its juices--smothered in gravy, Texas style!--for days now, and it has paid off. I think that instead of the dials for zoom and brush size, I will use some touch sliders. For the zoom, I want to use a strip touch sensor like this one:

I'm not sure about the brush size. I could use another strip, or a circular one. I think it might be good to use a strip. What I like the most about the strips is that I can touch at any point along it and jump to that point. Trying to map zoom levels and brush sizes to the 0-100 of the sensor is another issue, but not one that is insurmountable. This might also allow me to include a joystick for panning the image.

So that is going to require some modeling changes to the shell and, more importantly, to the PCB. I think I'll keep a backup of the current version just in case this turns out to be a bad idea.