I don't know that the rotaries can carry more than 8x twin rails.
I would not want my B-21s producing numerous, obvious IR events within BVR AAM range of opponent fighters any way. Perhaps a rotary armed with a very long ranged ordnance like an air launched SM-6 or Long Shot specifically for AEW/tanker targets would make some sense, but predominantly one assumes B-21s focus on air to ground.