Ben's site has a feature that, I believe, has become available only since DL redesigned its seatmaps.

In short, you can use Ben's site to view seatmaps for DL Codeshare flights. You can't do that on DL.com, not even when making a booking.

In my case, I booked a ticket through DL.com on DL 7850 (KE 036) ATL-ICN. I had to call SMS to make my seat selection in Y, and had to convince the agent that she could actually do it. There was also some confusion about which seat was which, because KE's website doesn't show all of its 744 seatmaps (it doesn't have the newly reconfigured one, which often operates on the ATL-ICN flight). Anyway, if you plug in date, flight 7850, and the cities, Ben's site pulls up the seatmap as if it were ATL-CVG.

The only glitch was that in the last few days before the flight, the seatmap became unavailable, perhaps locked by KE or DL. And the word of caution against relying on seatmaps as a proxy for availability goes double for international codeshares. My ICN-BKK and BKK-ICN flights showed tons of availability (whole rows, even!) in the back of the 773. In reality, the flights were totally full, maybe oversold.