Joe,
That's not quite right... As far as I understand it, the spectral sensitivity of the sensor is the only thing that will determine the response curve coming out of the chip. At that point, the camera manufacturer will make some determination about contrast response curves, but that's about all they can do.
They could bake a bit of filtration onto the chip in the cover glass or where the AA filter used to be, for example, but that would be the only way to adjust the spectral response. As far as i know, Leica did not do that in theirs, which is why it needs a bit of yellow filtration to match traditional films.
In color chips, they use a pretty strong adjustment to produce a properly balanced file, (due to the filter factor to get the RGB information from each photosite). Plus, they then probably make a contrast curve adjustment to make a file that looks like we expect. However, as far as I know, that is done for the JPGs only, and the RAW files don't have that baked into them.
The exceptions to that are the in-camera adjustments for the lens apparently. The RAW file information is modified to correct for falloff and color shifts in the Leica cameras, and presumably in others like the a7/a7r that are capable of identifying their own camera lenses.
But the big point that you did pick up on is that people are going back to using color filters on the B&W cameras for that reason. The only way to really modify the response to particular colors with the B&W camera is filtration.
It's pretty clear that the B&W cameras are for a very narrow segment of photographers. You have to be pretty hard-core B&W to want a camera that only shoots B&W! I wouldn't own one as my only camera, but as a second, I would certainly consider one.
There are rumors of a B&W version of the a7r, so that may be a possibility, but for the time being, I'm happy with my color cameras, and shooting for B&W conversion in PS or Nik.
---Michael