That the white dial is effective for this watch, stark, it makes the hands pop and puts the day/night indicator on full display. To that end, I think a simple barleycorn guilloche that is radially arrayed from the day/night indicator would be very dramatic, and yet still understated. It would be in keeping with the austere German qualities of the watch. I looked at a tourbillon example that employed this technique, where you can see the barleycorn guilloche is almost cupping, or framing the tourbillon, which in this watch becomes the day/night indicator.
My photo editing skills aren’t quite good enough to make the guilloche wrap around the entire watch, but you’ll probably see where I’m going with this.

