I got the missing pieces for a PWM driver from Balzer Elektronik and Holzinger (for the power connector), and soldered it all up. Two of the three PWM outputs don't work, but since I only need one, I don't really care. I don't know why it's made with three outputs -- maybe just because they can. I got it hooked up to the extruder and powered it up... and it heats! Trying to measure the temperature with the Arduino failed, it looks like I need a separate resistor in serial and measuring in the middle (the thermistor is 100K but goes down to about 1k or less when it's warm). I guesstimate that the extra resistor should be about the minimum value that the thermistor gets to, since that's where I'll need the most precision.
The photo shows the heater part with the PWM connectors (red and black) attached to the heater wires, and two multimeter cables (red and green) connected to the two thermistor outputs behind the plastic. Orientation seems immaterial for both connections. My biggest random problem here has been poor connections to the Arduino. I need to find a better connector than where I solder a multi-threaded wire to a little metal pin and put heat-shrink over it.