Impedance will be a function of frequency, so one number is not a 'right answer'.
I still have to rig up such a device, but the basic idea is that you drive the output of the device under test through a measured resistor, and then measure both the voltage across the resistor and the voltage across the device under test. The ratio of the DUT voltage to the resistor voltage multiplied by the impedance of the series resistor is the impedance of the DUT.
What I wanted to add to the jig were some very large, high quality coupling caps in order to be able to measure DC regulators accurately without worrying about AC coupling the generator. So, I have a pile of 10µF polypropylene caps, an aluminum box, and some connectors waiting to be wired up, but I've been too busy.
However, this jig and a nice test set (like an Audio Precision) that can do sweeps will let you generate a pretty accurate impedance sweep with relatively little hassle. I still have to finish my version, but that's how it would work.