John
I'm trying to work up something as well, so I went out to a 777 with a bare headset jack and measured voltage supplied by the aircraft. 11.75 volts at both the mike and speaker leads. My old headset tester had a circuit board in it with a lot of resistors/etc on it and a 9v battery, but don't think it bumped the voltage up. I may just spend the $70 and get the unit from CTS, though I think it would be lots of extra postage for you in OZ.
It should be relatively easy to take a known good unit and hook up a 9v and see what you get.
I spent some years fixing DC headsets until about 10 years ago, and it was difficult to get parts from DC directly, though that may have changed. I'm about to find out again, however.