I had a 2000 Pathfinder with a rear liftgate opening window a window washer. If Nissan figured it out 25 years ago, Rivian should be able to figure it out now.
I've only had my R1S for a week and a shade is definitely needed since I park outside in FL (roof measured 165 deg. F one afternoon). Did you use the suction cup and magnet to keep the sag to a minimum?