That actually kind of exists currently via "MDCUtil", which should work on both macOS and Linux:

https://github.com/toasterllc/MDCCode/blob/rev10/Tools/MDCUt...

For example:

    $ MDCUtil ImgReadFull <image number> output.cfa

That outputs the raw data stored on the SD card though, so I need to implement converting that to a standard format like DNG.