I called the AY customer service desk - only a 10 minute wait and spoke to a switched on lady. I asked for the ghost PNRs associated with the two codeshares and she said
"ah, you will need them for seat reservations - please wait a minute..... 2 minutes on hold..... here they are sir!"
What a pleasant CS experience, I have now logged into MMB on both CX and QF and reserved seats.
Well done Finnair CS.