I built a second copy of the adapter for my other meter - and guess what? It didn't work. In fact, the second meter didn't work with the first adapter, either (should have checked this before). Turns out I had mis-interpreted the analysis of the interface circuit inside the meter, and come up with a bad solution to interfacing the thing with the nano. I re-worked the interface so it's a lot more robust with both meters, and updated all the project text and images (this took much longer than the actual fix).
Anyway, it works well with both meters now, and the electrical interface makes a little more sense.
So much for my hack-a-day rate. I'm down to a hack-every-two-days now. :-(