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With your help, a voltmeter, etc. I figured out the Lennox Harmony II well enough to retrofit a signaturestat without problems. I now enjoy enjoy timed air circulation, dewpoint-adjusted humidity control, etc.
The hardest part was not the wiring, it was fixing the 3"+ long holes in the wall where the original HVAC contractor had installed the horse-hair humidistat. Patching the holes was one thing, getting the color on top to match the adjacent sections was a two-day process.
I have yet to undo some of the other creative wiring this contractor did so that the Lennox panel is not backfed 12V and 5V on Y1 and Y2 whenever the humidistat is calling for heat. I'll get to that later in the season.
So, thanks again!
PS: The variable speed function on the air handler seems to be determined by differential voltage...