Am I missing something here?
The answer is as simple as digital photography. You do it electronically. The sensor doesn't 'know' what colour to record, it has to be told by the firmware. In the same way you can adjust the colour balance, why not adjust everything else? OK, it would add a lot more controls to our, already overburdened camera bodies, but if that's what you want I'm sure someone will build it. Grads, Spots, Diffusers etc. are all perfectly simple.
The real project would be to figure out how to do a CPL in firmware
