What's written in the wiki worked today. I have a UA ticket with an NH leg listed as a UA codeshare; the UA agent changed the UA codeshare in the record to an NH flight (this took 10 minutes and some persistence from me for the agent to realize that this was necessary and to do it). After that, when I pulled up the flight record, chose "Upgrade your trip" and chose the NH segment, it showed "no upgrade available". But, when I went to https://www.united.com/loyalty/partn...points/request instead, I WAS able to get the upgrade request.

So, once the UA codeshare flight has been changed to an ANA flight, the specific way that you request the upgrade matters.