Something that may or may not be useful depending on what people have on their bookings. I flew an Aeroplan ticket a few months ago that had a 4Y segment, followed by LH, then TK (stopover in IST after the LH flight). The AC website wouldn't let me select seats on the TK flight, but the LH site did.

I was also later able to change my seat assignment via the TK website, so perhaps my booking was not impacted by the issues that some others have experienced. But if you have other *A partners on the booking, might be worth checking to see whether any of their sites will let you select seats on your TK leg.