There is. But, since they've reverted back to parceling out more Premium seats, on some planes, the "Coach Choice" seating isn't really worth the extra money.

You can always try to snag a seat through OLCI. Depending on the loads, you may be able to select a P seat.

Otherwise, I'd keep trying with NW, not DL, since it is a ticket operated by NW. Whenever I need to get premium seating on DL using my NW status, I always call DL, and it works vice versa.