With WindowManager.addView and WindowManager.LayoutParams, a view can be added to the screen with TYPE_SYSTEM_OVERLAY, which will then be drawn on top of all other applications. This can be used for various effects, such as colouring the screen or drawing a view which will always be visible. However, I haven't found a way for this overlay to draw over the background that's drawn behind the soft keys.
With the new options to draw in that area supported by KitKat, I figured I could try FLAG_TRANSLUCENT_NAVIGATION and FLAG_TRANSLUCENT_STATUS, but the background is still off limits. FLAG_LAYOUT_IN_SCREEN and FLAG_FULLSCREEN don't seem to help. It seems that the only way to affect the soft key background is by using FLAG_DIM_BEHIND and setting .dimAmount, but this can only be used for dimming the soft key background. Is it possible to actually draw something there, the way it is from inside an activity? If so, how could it be done?