Last summer I flew MSP-AMS-OSL-AMS-MSP. The TATL flights were DL metal; the European segments were on KLM. After I booked my ticket I called the PM line about my intraEurope KLM seat assignments. The agent said that there was a new special phone line on which she could call to get seat assignments. She put me on hold, made a call, and secured my seat assignments in Economy Comfort on KLM, as I had requested. As a PM I was not charged for the EC seats. The DL agent also gave me my KLM confirmation number. I then went to the KLM website and viewed my seat assignments, which were the same as the DL agent had given me. Subsequently, when a slightly better EC seat became available I was able to change it on the KLM site.

I don't know if this DL-KLM phone line still exists. From reading the FT posts, I think I hit some sort of perfect storm of good communication between DL and KLM!