The DL/KL situation seems to get worse (and weirder) by the day. I think the advice in your situation would be to call KL, preferably their actual Dutch offices. You can also try Twitter.

I had the opposite problem: DL's site let me choose EC seats on KL for some flights last week, but then at OLCI KL wouldn't let me have them since I'm just GM. That was a first, as I've always gotten EC within EU for free on KL, and been able to choose those seats on DL. As DM you're actually entitled to them, but now can't choose them....