Hello Hartington. For what it is worth, I have a case that is perhaps similar enough to yours: I booked Finnair GVA-HEL-HKG-SYD-BKK-HEL-GVA in J-class. The HKG-SYD and SYD-BKK sectors are code-shares operated by CX and BA respectively. On the Finnair website I could select my seats for the Finnair and CX flights; for the BA flight it seemed to be possible in principle but kept giving an error message. I called Finnair reservations and they were able to get the seats I wanted - but took them a couple of days to confirm (I guess they have to send the request to BA). There was no charge for the BA seat selection, but I think I made the booking before BA introduced its new policy, so perhaps the situation has changed.