Update: had another go at it last night and I got it! As others said, it just takes a ridiculous amount of effort to make it move. I wedged the pin socket / breaker bar in on the back side, against the trailing arm, and then I used a 3' wrecking bar across two lug bolts to get it started.

Getting it to yield initially was the hardest part, which is consistent with how I would expect steel to behave in a failure situation - it took an insane amount of torque to get it started, and only a ridiculous amount of torque to keep going once it had yielded. I went through an iterative process - turn the hub a few degrees, check end play - until I was down to about 0.002", maybe just a little under.

Thanks for the suggestions and advice...not a fun job, but I've done worse!