I trying to do the same thing you are basically and have a few questions, maybe you can help me based on your research:
1. From what you said above, it seems the 12V(old firewire) and 5V(new usb) share the SAME pin on the actual dock connector. If this is the case, then what you said should be true, all we need to do is cut open the Pioneer Ipod cable, find the 12V wire and either regulate it down to 5V, or splice in a separate 5V source.
Can anyone confirm that the 12V and 5V pins on the connector are indeed the same?
2. Did you identify the actual 12V wire in the bundle of wires