Thank you for the post. Very interesting. This is something that was in the back of my head to look at for a while but never took the time. To me it makes all these discussions, not to mention the concept of 4, 5, 6 figure speaker cables rather foolish.
Given the electromechanical nature of the drivers, it could be difficult to measure the electrical resistance of the driver(s) in circuit during operation. Perhaps as an academic exercise you could limit the frequency of the incoming signal to say 50 or 60 Hz, and then use a small DC stimulus to measure the DC resistance. You would be severely response limited, perhaps 100 millisecond rise time. I expect someone has given this a lot of thought and has a better method.
Also interesting about the speaker becoming a different speaker at different driver levels. There was an interesting post on ASR recently where a person was "fixing" a commercial speaker. He did response and impedance measurements at multiple drive levels. There were some significant differences, though less after his changes.
You are convincing me more and more that active is the way to go long term.