The DSP, a philips 7705 car audio processor,

Ah! Now this explains a lot. I didn't know that Philips was making DSPs...

talks to the SA1100 via I2C for control and I2S for raw audio data. The data to the
DSP from the SA is output only, ie we can send data to it but not get it back. The tuner outputs a raw fm multiplex signal
directly to the DSP, which then processes it, extracts the RDS data stream and the audio L/R signals, and passes the audio
straight through to the onboard DACs. There is no way of intercepting it enroute. The only way of recording the radio would be
to sample the audio outputs with a separate ADC, such as the one the mk2 board uses...

Does this mean recording radio might be possible with the mk2? Is there hope?

BTW is there a reason for using a car DSP, other than convenience? Is it cheaper than, say, a Motorola 56600?

When asking my original question, I assumed that the architecture resembled this: the tuner module outputs audio L/R and RDS data separately, the audio goes through a ADC, then through the SA1100, then through a DSP, then through an DAC. Would such an architecture be more expensive, or just more difficult to implement?

Thanks for your patience.