Detecting parasitic drain with a multimeter?

Obviously one can measure actual current, if it's possible to get the leads in all the right places.

But will measuring impedance also give hints as to how much (if any) parasitic draw there might be?

I tested the tail cap of my Warrior X Pro and got infinite impedance without pressing the button, and then two different readings depending on how far the button was pressed, due to its gas pedal UI.

Can the same trick be used to measure across the contacts inside the tube, since both positive and negative are at the head thanks to the strange proprietary 21700?

You have to break the circuit and use the meter probes to reestablish the circuit and with the light not turned on use first the ma scale and then the microamp scale if you don't get a reading. Acceptable drain is in the microamps and calculated should give you storage time of at least 3-5 years before dead any less and you either need to be using the light (and recharging it) often