....An optical reducer is something I have long thought was possible on a DSLR and wondered why nobody had made one....
I had often puzzled over the same question. You can actually already get a Reducer for cinema lenses, Vantage have had a (expensive) 0.7x PL mount reducer available for years
http://www.vantagefilm.com/en/news/news_2002-10_01_i.shtml
But it isn't general purpose, it is only appropriate for long focal length lenses. I think this must relate to the same issue that Chris Mann identified. If the light exiting the lens is almost parallel it isn't too much of a problem if you introduce a reducer, but that won't work for wide angle lenses without losing infinity focusing.
Maybe metabones have added a collimating lens ahead of the reducer in a similar way that Canon and Fujinon do in the extender section of ENG lenses. ENG lenses used to come with 0.8x reducers built in working alongside the extender, to reduce the image circle when cropping a 4x3 sensor to 16x9. There the reduction happened in that collimated space.
I find it difficult to believe there is a conspiracy to deny us photographic reducers, Maybe it has just been too expensive to do so with sufficient quality. It will be interesting to see what the quality is of the image from the metabones adapter. I would love to see a test chart.