RGB 2 Black Only

It's not possible to create an RGB to Grayscale profile.  However, you can convert from RGB to CMYK where there is no CMY in the 4/c image.  Attached is a profile that may be a good starting point.  It was created by going to the statistics tab and sorting by dL.  Copied those numbers, inverted them and then pasted them into the black channel.  Then deleted the CMY values.

With the attached profile you can of course adjust dot gain curves and or color correct to suit your preference as with any RGB profile.