First, the rear canti bosses don't take on the color of the stays in "stays" color schemes, but rather they remain the main color (i.e. "1st"). This does not happen with schemes like "spear" and "two/three color angled."
Second, when modeling centerpull brakes with cantilevers I found that values of PY less than D/2 create a blank space in the middle of the canti pivot. I expect this is because the arm is drawn from the top of the pivot regardless of the PY value. An easy way to allow weirdos like me to model centerpull brakes would be to give extensions above and below the pivot their own dimensions and ignore values for those dimensions less than D/2.