Darktable can use an icc (icm) camera profile from other sources, e.g. created with DCamProf/Lumariver, or created within Capture One’s Color Editor (both are LUT profiles). For Nikon cameras, the free Capture NX-D creates temporary .icm profiles when you open a NEF file with it – you can copy it to the color/in darktable directory. Then you need to do three things in darktable: reset the Base Curve to linear, pick the icm profile from the Input Profile module, and then use the Unbreak Input Profile module. For my camera (Coolpix A) the correct unbreak settings are: linear (0), gamma (0.4500). You’ll have your Nikon colours to work from, as opposed to the default dcraw-based matrix profiles.
The Capture One profiles require a different unbreak input profile settings: linear (0.0500) and then I had to come up with a nice base curve (I modified the Neutral preset in the darkest area, but that’s a camera-specific and slightly subjective setting).