I agree with asking Luft for the AC flight confirmation code. We have done the reverse as well. Booked my husband on AC but flight was an AC/Luft co-share. AC gave me the Luft confirm number and I was able to book seats for him. And I seriously begged that he not be given a middle seat.

Note however, that this was only for the Toronto to Frankfurt or Munich etc. part of the trip.

Since we were travelling onwards in Europe - but on Luft inter-European flights with AC codeshare flight numbers I was not able to his book seats.

He was SA Gold - but that only helped when it came to luggage, lounge access etc. - seats on the inter-Europe flights booked in Tanglo/Flex (or their version of it) could not be booked in advance. Nothing like receiving a confirm of your flights along with a notice about bumping to make you feel secure. Since I was travelling on a Business reward ticket, well I had seats the whole way!