After a bit of research, I can answer my own post somewhat.

The knock sensor common to all the M104s and several other engines is a broadband piezoelectric device. It is not tuned to a specific frequency of vibration, but as a broadband device, it responds to a range of vibrational frequencies so Mercedes can use the same one in many applications. The sensor itself is made by Siemens of France.

The knock sensor sends a voltage it generates from its piezoelectric properties to the engine computer, and the ECU interprets the voltage and retards timing appropriately. The ECU does not send a voltage to the sensor, so therefore the ECU has no knowledge of whether or not the sensor is even connected, (slight amendment: a completely disconnected sensor will set a code in short order) let alone when it goes bad as the no-knock condition results in zero voltage sent. There is no "live zero" to indicate failure if absent. No code can or will be set for a non-functional sensor. A code is set when the sensor asks for maximum retard on repeated occasions.

All the dealers I stopped at on my way up north the other day to try to find one claimed they had never sold one. I had one ordered and picked it up today ($74 CDN). I changed it in the back of the dealer's parking lot in less than 5min. Problem solved. My C36's engine is quite noticably quieter mechanically, and doesn't knock under load any more, although I suspect I have taken a slight hit on fuel mileage from what I used over a few hours after installing the sensor, but that is to be expected. Possibly the problem is not restricted to my AMG, just more noticable due to the tuning of the engine. I think I'll put a new one on my wife's C280 as well.

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'96 C280 (gone)
'97 C36
'05 C230k