Thanks guys, good information here.
I don't have all of my books handy, but I did find a section about it in my "big load" dryer service manual. It looks like a clever design, as the buzzer parallels the timer, but is in series with the motor start switch, so it's basically the spin-down of the motor (via the centrifugal switch) that times the buzzer's buzzing duration. Likewise, the repeated chiming of the bell on the electronic control dryers in the permanent press setting is basically a clever usage of a side effect of the capacitor charging/discharging when the load is dry.
The book also has a chart, which answers part of my original question:
* DE91 and DE/DG 106, 107, and 308 do not have an end-of-cycle signal.
* DE/DG 309, 408, and 409 have an end-of-cycle buzzer.
* DE/DG 608 and 808 have an end-of-cycle chime (the bell).