Yes, I selected seats in mid-Sept. for a 12/18 KLM flight, but the interface is flaky. For a couple days it would only let me choose a seat for my wife but would ignore any seat selection for myself. Eventually it let me choose both.
The GE line could not access KLM seat selection at all when I called them (KLM system was "inaccessbile" at the time).

You have to use "nwa.com\Reservations Center\Manage My Reservations\My nwa.com Itineraries". The "View/Change Reservations" page will not allow seat selection on KLM.

Somewhere I think I also have a phone # KLM emailed me. I'll check later.