First of all, Welcome to FT!
As far as I know, you can not buy premium economy on any carrier through UA. Since they don't sell it themselves, their system really just isn't set up for it.
AC will be unable to let you buy up to their premium economy in advance. For one thing, they will not have control of the ticket until the day of the flight (and the flight on AC), so wouldn't be able to make any changes.
Even if they could make changes, with the ticket on UAs stock, they wouldn't be able to tie any purchases to the ticket. Its the same reason why, if you buy a ticket through UA that books into Tango class on AC, even AC will not be able to let you buy a seat selection or any other of the other ancilliary items (pre-buy bob, etc.), or allow you to purchase one of the seats closer to the front that cost more.
If you want to buy into premium economy at time of ticketing, unless there's some other way around I don't know of, you'll need to buy the ticket with AC. Alternatively, if seats are still available on the day of, you may get an offer to buy up, but of course, not guaranteed that there will even be seats in that cabin at that time.
And just want to correct your terminology - I believe you are referring to AC PNR in your last paragraph.