Has anyone had success buying a Premium Plus upgrade, either at check-in or earlier, for the 2nd leg of a trip booked via UA?

IAD-HND is in Premium Plus. The Economy leg is on ANA (HND-BKK). This is all on one ticket. Don't see any upgrade options for PP available either through United or ANA for the HND-BKK leg. Never flown ANA before, so not familiar if they offer PP.

IAD-HND is a 777-200 and HND-BKK is 787-9. I'm fine with the economy leg, but was wondering if a PP upgrade is even possible when the classes are split like that.