One way I could see you being able to do this would be by using the Mic input on the Empeg/rio Car and a clamp-on type 'handsfree' speaker/microhpone ala some of the cheap handsfree kits for cellphones.

Connect the output of the handsfree microphone to both the Aux-input(s) (for listening) and the mic-input (for triggering).

Then using some mic sampling software in the empeg (which would have to written/obtained), have it sample the Empeg mic input line for speaker output (i.e. remove the white noise element), when valid sounds appear from the Ipaq (e.g. its speaking):
Switch the input to the Aux input while the sound plays then switch back shortly after the sound stops.
For ease of testing/mapping via hijack , the 'switch to aux-input' could be handled by faking a 'cellphone mute' input event and letting hijack mapping handle the rest].
the sampling software may already be in progress - someone was working on a app using the mic as a input to help set the EQ correctly in your car, so this software is probably already around and can be modified slightly for the triggering].

With a good seal on the Ipaqs speaker 'grille'/ sound output area, the leakage of noise from the cars interior into the sampling mic input should be reduced, the software could be set to ignore white noise below a certain level and to trigger the aux-input only when the Ipaq is speaking.

With some good design the system could work somehwat seamlessly in terms of switching etc.

The issue then would be connecting/covering the Ipaqs speaker output in a clever (in a practical and aesthetically pleasing) way and having to accept a "cable" connection to the Empeg for sampling and playback purposes.

I assume the Ipaq has no 'line level sound' output connection - only the internal speaker output. Which AFAICT is the case with Ipaqs.