I don't know about tweeting KLM re: AF metal, but I have had good success with twitter direct messages to KLM for KLM operated flights and AF for AF operated flights. Both have US-focused twitter accounts that seem relatively adept at accommodating things booked on 006.

Best thing is you don't have to sit on hold waiting for a phone agent that may or may not know what they're doing.

Note that all of my AF/KLM flights have been in either J or PE with STE+ status. As others have said, I believe that makes a difference as to whether you can select seats at all on even long-haul flights.