How can I manage online a booking for a Malaysian Airlines flight issued by British Airways?

Using some BAEC Avios (the British Airways Executive Club "miles"), I have booked a flight with Malaysian Airlines, who are a OneWorld partner of British Airways (BA). The ticket was issued on BA (125) stock, for the Malaysian flight with a MH flight number. BA have given me a 6 character Amadeus reference, and a 5 character Malaysian reference for the flight.

If I try to use the BA Manage My Booking, with the Amadeus reference, I get the error "Sorry, we are unable to add bookings that do not contain any British Airways flights to your account.". This error might make sense, were it not for the fact that it happily lets me manage BA ticketed flights with Cathay Pacific and Qantas. Clearly there is more OneWorld ticketing integration on the BA wesbite than the person writing the error message knows... But apparently integration with Malaysian isn't one of the supported one.

Next up, I tried the Malaysian website, and tried to use their Manage My Booking with the 5 character reference I have. That gives the error "We are unable to process your booking online. Please contact our call center".

Finally, I tried with the FinnAir website, as I know they are able to manage most OneWorld airline bookings, often even in cases when the ticketing or operating airline won't! FinnAir is able to show me the details of the flight, which is a good start, but sadly isn't able to let me select my seat.

Am I falling down the cracks between not fully integrated OneWorld ticketing systems here? Or is there another trick to let me manage such a booking online? (I've never had much luck with phoning Malaysian Airlines in the past is all....)