When flying in the J "Cabin", even in Intra-Europe, I've always been able to select seats on KL, AF, etc. However, Y seats, not so much. I'd just type the KL ticket into KLM.com (or AF.com) and pick what I wanted. It's been that way since the NW days.

Posters are right, there's no way to use DL script to upgrade. In the event of an OP-UP they will bring you up after they've cleared all their elites. Mind you they have far less elites because they don't give away status earning miles like Candy over there.

Be that as it may, the only way KL would accept DL upgrade scrip is if it they could get $$$ for it and it wasn't any better or worse than their own FFP. I don't see either happening.