I'd be really interested to know more about the optics in the adapter. Looks like an interesting solution. But it would be good to know how much light loss, and how much the image circle is actually expanded.

I bought an adapter on ebay without optical correction and mounted it on my gh2. I had to use the 2x extender in the camera. The all thing worked but I'm not satisfied at all on image quality. If you open lens too much you get a blurry image. Besides that you have to work with no optical stabilization, and with a 2X focal lenght. Maybe on bigger sensors works better, but I wouldn't suggest it for m43.

Light loss would probably be double if it is a 2x. For the full frame, probably more.
Would like to see some samples from the BMPCC they are using.
You could run audio recorders like the Tascam DR-40 from the USB port too I bet.