So this wound up being kind of funny. I checked everything, and I mean every part of that side. Used multimeter resistance mode on the transistors to double check them and compared values to the good side. Everything matched up. All the top trace stuff to ground went to ground. All terminals were fine with or without wires screwed in. The couple little bridge traces were fine. So left with nothing else, I went trace short hunting systematically, every point to every other point.
Then I finally found the culprit.
The last time I had replaced the 10m90s for that rail, the solder pad for one leg lifted up just a hair and I had pushed it back down and soldered everything up fine. But moving the board around must have snapped it somehow, or maybe it happened when I double checked the tightness of the screws on the heatsinks.
What it left me with was a shorted trace that you couldn't see at all under even magnification. Ridiculous. Took less than half a minute to remove the busted loose pad, score a bit of the trace down to copper, tin it, bend the cut lead over it and solder it properly.
Then everything fired up, just as it should.