So, my recent experience on this with an upcoming itinerary that is a mix of AC/NH/UA segments, all marketed as AC except for one NH marketed leg, is that I was able to use the Amadeus PNR (which is what AC uses), and when I typed it in, UA was able to pull it up, and displayed the UA PNR (I could not find a UA PNR on the AC site). I had to do this because even though I booked with my MP number on the itinerary, somehow, that didn't transfer over to the United PNR, which is why it didn't auto-populate in my MP account, like it should have (also explained why it was trying to charge me to select preferred seats on the UA segment, when I should have those for free due to my silver status).

What I can't figure out is the NH flights. AC let me pick a seat on SIN-NRT, on a AC-marketed, NH-operated leg, but not on the NH-marketed and operated leg (NRT-SIN). The seats that showed available were only in the very back section, except for middles toward the front of Y section. NH site would allow me only to select NH-marketed leg, again, only in the back section, also with a note that I could not pay for chargeable seats, since the ticket was not an NH-ticket (its an 014 NH ticket). Apparently aisle/window seats on NH that aren't in the back few rows all have a fee, at least on lower fares. So I guess I'm stuck in the back part of Y, maybe until online check in, when those seats may be available to select.

On that note, anyone know if AC online check in will allow me to select seats closer to the front on NH(-marketed) flight when I check in, if available, of course? Have not done AC online checkin on a partner for a long time, so not sure if the seat map will be available on partner flights, or if I have to deal with any change at the NH desk in NRT to see if we can get seats closer to the front (I'm guessing a lot less of a chance at T-3 in NRT then if I can do it at OLCI).