NSA-proof iPhone calls to dead anarchists

This requires the baseband processor to have control over the audio codec which it may or may not have based on the phone that you have.

Of the baseband processor has direct i2c/spi and direct i2s access then, yes, it is possible. If it doesn't, then RCE on the baseband processor is defeated with Signal.

The overzealous HN title will no doubt be updated shortly (currently says "NSA-proof iPhone calls to dead anarchists", should say "Free, Worldwide, Encrypted Phone Calls for iPhone".

I highly doubt Moxie / WhisperSystems thinks this "solves" the problem, as opposed to reducing attack surface in important ways. RedPhone and Signal address an important part of the convenience-vs-security pareto curve. Tapping communication lines is a whole lot easier, and harder to detect, than exploiting hardware.