I just noticed this thread I started has been added to. So I should update what I did for this trip back in October 2023.

As I have Flying Blue Platinum status, I called the FB Platinum Service Line even though Flying Blue had nothing to do flight this ticket. The agent could not select seats as the Flying Blue system is different to the MU system. However, he went to so far as to contact other offices and managed to get an English speaking MU person on the phone who booked my seats - I don't know how he did this or if this will work for everyone but that's what happened to me. BTW, the MU London phone number is a waste of time and effort, even the FB agent had no luck with that one.