Taking photos at ISO 409,600 would be pretty awesome IMO.
If you can do that already with the sensitivity you've brought, higher ISO doesn't buy you anything...
Why not use the Sony A7S? The photographer uses a Canon 200-400mm f/4 lens, and the nearest Sony equivalent is 70-400mm f/4-5.6, which means at the long end it captures only half as much light, instantly negating any possible ISO range advantage offered by the A7S.
Myself, I'm using my grandfather's old Canon 50mm/f1.4 on a Canon Model 7 rangefinder (essentially the last battery-free Canon) for film and the same lens on a (used) Leica M8 for digital. Nice fast lens, but still in 35mm-land.
And it's very expensive, which means even a relatively small advantage in image quality will make the resulting pictures more valuable, at least for a few years until someone else does this again with newer technology. DSLR photographs are truly commoditized now, but medium format are not, never have been, and probably never will be.