I had a similar experience about a year and a half ago with KLM seats on a leg of NW WP award travel. I was going SEA - IAD - AMS. Could not get a specific seat assignment on the KLM leg (IAD - AMS). I was worried because I didn't want to get stuck in a 747 middle seat in coach on that long flight. Ultimately there wasn't anything I could do about it. I requested a window+aisle, but they wouldn't make a commitment to any particular seat. It worked out OK (I did get the kind of seats I wanted) but I did worry about it until my ... was in the seat.

Good luck, and have a great trip!