I tried the online tool last night. It accepted my itinerary and I thought all was well. Turns out that it queues to LH and LH would only sell it to me if I was on an LH intercontinental flight into or out of Japan. I'm not.
I called UA this morning and the agent tried to process it but the fare requires M inventory and that was not available on either of the segments I want. I'm going to keep an eye on it for a few days but it is looking more and more like I'm going to have to buy these tickets outright.
What is involved in upgrading to premium on NH with the Japan Pass? Is that something you do in advance or only at the airport?