What you're describing can be done much simpler, easier, and more reliable, with a simple resistance mixer circuit, after the output of the empeg and before the amplifier.

The external mixer circuit would also meet your criteria of wanting the directions from the IPAQ playing at a constant volume if you turn down the player volume. Doing it in software with the aux or mic inputs would mean the ipaq gets turned down with the empeg.

There is currently no software to do what you want, and a lot of coding work would be required to do that job. So it would be a while (if ever) before you were able to do what you want in software. The external mixer circut, on the other hand, could be done right away.

Anyone have a link to a schematic for a simple mixer circuit?

On the other hand, a generic "blend" feature, taking the mic, aux, or Tuner audio inputs, sampling them and blending them with the output, would be totally cool for posterity, and for other projects like this. Wonder if that's something that could be built into hijack? Dunno which sub-permutations of that are even possible, there's some limitations in the DSP and you might not be able to sample the aux input at the same time as you're playing audio, for example. But you could probably do it with the mic input at least, if nothing else.