You replace the Mosaic filter (RGB) with plain glass. Sensors are monochrome sensors, sensitive into the near Infrared. Mosaic filters are used for Color and IR absorbing glass is used to get rid of IR. Between these two filters, 3/4 of the light is absorbed before it hits the sensor. So- a visible-only Monochrome sensor has twice the ISo rating of a color sensor, and as much as 4x the resolution- when the object is all red or all blue. My first "store Bought" digital SLR from 1992 is a monochrome IR camera, special ordered from Kodak. Made on request.
The first digital sensors that we made in the early 1980s were two-color, but we used two detectors made from different material. Each of the sensors picked up a different portion of the spectrum, color without a Mosaic Filter.
I wasn't interested in a B&W Leica when I first saw this thread but I think your post just gave me an orgasm. I hate to admit this but I'm getting intrigued.
Apparently March 10th may see the announcement of more than just a b&w Leica:
Mirrorless Rumors | Blog | New whole range of Leica products to be announced on May the 10th?