I ran into a similar issue a few months ago trying to use GUCs to upgrade JFK-AMS-DXB and DXB-AMS-EWR, with all but the AMS-EWR segments DL-coded/KL-operated. After 6 phone calls to the DM line, I did some digging around on FT and found an old thread complaining about frustration using GUCs where someone suggested calling Singapore reservations (you can find the number on DL.com).
Sure enough, with just one phone call (and some time on hold), the agent understood my request and was able to apply the GUC to the KL flights where Z class was available. Might be worth a shot!