I am in the same boat as the OP and I don't think anyone got to the actual answer (which may mean that the answer is no!).
I'm DL Platinum, and I've got an AF-ticketed, AF-coded ticket, MSP-LHR-CDG-MLE. Trying to upgrade only the first segment which is DL metal, MSP-LHR. (Delta didn't offer any one-way itineraries to Maldives, so I had to buy from AF). Just FYI as a DL Plat on a DL metal, AF-coded ticket, you are unable to even get Comfort Plus ahead of time, just at check-in.
Sadly, on a DL-ticketed, KLM-metal flight, I think I'm able to confirm Comfort Plus at booking. Correct me if I'm wrong!
That must be related to DL also not offering Premium Select or Delta One cash upgrades on my app; my flight is Tuesday and there are more than half the D1 cabin seats showing available. Sure, some may be sold but that's a lot.
Does anyone have any experience with DL selling upgrades at checkin with an AF or KL ticket stock? (I was offered and took one at OLCI on ATH-JFK 6 weeks ago for $699 but it was a DL-ticketed flight).