That solution appeals to my inner geek, but can give a false sense of security. Finnair's system may not accept all, or indeed any, of the allocations made through third parties. When i called Finnair the seat hadn't made it over to AY from MH, although they did show up on BAEC's MMB. I have a feeling Finnair's IT could give BA's a run for its money in terms of crankiness.

Calling or web chatting Finnair to make the seat allocation seems the best way of handling it: it's simple enough if you have the patience to wait or to fight the chatbot for human attention