I've not heard of any Medium format / video hybrid to date. While it is not to say that we won't see such a technological move in the future, right now the 35mm Digital Still/Video hybrids are a result of news agencies desiring more capability out of their field photographers at press events, junkets, and the like.
The 5D MkII and 7D from Canon were not intended for independent filmmakers, it just happened to be prove too much of a cost effective tool for them to avoid. The medium format market does not have the same possible advantage given that a digital back for a medium format camera that only does stills is well outside of the price range that indie filmmakers would even consider. Somewhere around $17,000 I think? Truth be told it has been a while since I have priced out digital backs for medium format cameras.