Is it so important to have an instant return mirror in medium format anyway? You're never going to be machine gunning with them...
I have never felt the lack of an instant return mirror has been a problem. It would be nice to have but not at the increased weight of an electric motor drive. I would rather use the weight to carry more lenses with me. I never had a problem with the lack of the gliding mirror system on my Hasselblad 503 CX when I use the 250mm lens, 500mm lens or the 500mm lens with the 2XE.