OK, here's what I've now determined:
1) The signal out of the HU varies in behavior, depending on whether the factory amp is plugged in or not.
When the OE amp is plugged in, the signal out of the HU is fixed in level.
http://kward1.homestead.com/300chufixedop.html
When the OE amp is unplugged, the HU seems to "see" this, and the output is now varying with the volume setting.
However, whenever the analog audio lines from the HU to the OEM amp are cut loose, the HU stops passing the Nav voice or the Uconnect voice through to those outputs. We didn't need to have the whole amp unplugged, simply cutting the four audio lines was enough.
Also, in answering my above question, the Nav voice output and the Uconnect voice output go from their modules into the HU, on different pins, and then are passed through the outputs of the head unit.
As near as I can determine, the CAN bus controllers "see" the disconnect between the HU and the OE amp and signal for the HU to change behavior. The HU does still mute though, which obviously makes little sense.
2) In either mode the output of the HU is very high in voltage. No line drivers needed here.
3) The frequency response of the HU line output is flat above 20 or so, and below 20 or so has some sort of auto-loudness feature that at lowest volumes results in about an 8dB tilt from the bass to the treble.
http://kward1.homestead.com/300CHUOP.html
4) The F door speakers play from about 120 all the way down. The front dash speakers play from 100 and up, with a significant rolloff at about 16kHz. The R deck 6x9s play from 100 Hz and up with a slight rolloff at about 18k. The R deck "sub" plays from 50 and down and is the least-eq'd signal from the amp.
(Note: The above has been corrected to properly describe the front dash rolloff in the highs...)
For pics of all but the sub output:
http://kward1.homestead.com/300campop.html