PhaseOne sell a 100MP IQ3 B&W digital back, which is basically exactly what you're after. The quality of this is brilliant - I've seen some really astonishing

PhaseOne sell a 100MP IQ3 B&W digital back, which is basically exactly what you're after. The quality of this is brilliant - I've seen some really astonishing work out of it - but it might be out of price range a bit - you're looking at north of $50k for a working setup, so many people just rent them when needed. There might be some B&W scanning backs that are a bit cheaper, but those don't really work too well if your subject is moving.