Alright, got it about 95% removed, so unless you know it's there, it's not really noticeable. I'll leave it at that.
Called Katzkin, but all they would say is "We recommend Mother's products." So I called Mother's and talked to one of their sales guys. He recommended their foamy leather wash (the leather cleaner they sell is apparently of industrial strength, so that would have been the last resort) to be followed by their conditioner.
I called the local Katzkin installer to see if they had anything at their shop or would recommend something. They went old school and also mentioned that hairspray could be used to removed ink from leather ("...back in the days."). They also recommended Magic Eraser.
So I gently used the Magic Eraser for some base work and then went back to the Zaino leather cleaner. What makes the difference is using a toothbrush to massage the cleaner into the leather; before I just sprayed it on and rubbed/wiped with an old t-shirt. But with the toothbrush you actually get a nice foam going that seemed to lift the dirt. Follow up with Zaino's Leather in a Bottle, and it's all good.
I may have lost some of the texture of the leather from the Magic Eraser, but perhaps I am just hyper-sensitive at this point. In any case, give it a few more months of normal wear and tear and you wouldn't see it even if that was the case.
Thanks for everybody's advice. I would have tried the Leather Magic next, but given the holiday and the shipping charges I wanted to try the other stuff first...