I did. No problem there. I simply routed it along the rack stay and then along the rack itself. Attaching the cable in strategic places with black tape, leaving enough slack to ensure folding will not pull the cable. I also wrapped some black tape around the cable in places where it might rub against things. I removed the rack reflector and put a standard format taillight there instead (5 cm bolt distance). Those lights have a reflector in addition to the light, so it's all good. There is also a reflector on the mudguard, but the more the better.

If you are going to use panniers with hooks that wrap completely around the rack tubes, you may have to adapt the cable routing to that. Should not be difficult, just add slack sections in the right places, or route it along the middle bars.

Busch + Müller have several dynamo taillights with brake light function. They depend on the dynamo pulses to function, so according to what Jipe has said they will not work if connected to the Supernova headlight, but they will work with most other headlight brands and also if connected directly to the dynamo. Example: "Busch + Müller Toplight Line brake plus". I use this one on another bike, and the effect is very visible.

I don't see "small" as a positive feature for a light. In my view the Busch + Müller My has the same problem as the original supernova taillight: It is tiny, and has no reflector. Sure it may be bright, but regardless of brightness, a visible size has advantages over point-shaped lights, in helping other estimate your distance/speed.

Beware for any light mounted down near the hub axle, like the original supernova: There will be certain angles where the rim/tyre blocks the light, making you hard to see from that direction. The rack or seatpost are safer locations, in that sense.