Thank you TheDrPhil and flyerCO and the rest of the thread for all the pointers.
This morning I successfully upgraded AF economy tickets on a 057 (AF marketed/AF metal) itinerary (IAD-CDG-MLE/MLE-CMB-CDG-IAD). Although I had a travel agent tell me that he had confirmed with Delta that GUCs could only be used on DL marketed partner flights, I called the Diamond line and the first rep I spoke with said no problem. While I was on the phone with her, I went to AF's website and purchased two round trip economy (N) tickets and then gave her the AF booking reference number, she saw it immediately, but when she tried to call the AF desk, the call dropped. I called back and despite the impending nor'easter, got a different agent in a few seconds. The first thing I asked both reps was if they knew how to apply GUCs to AF tickets and they both said yes, so it would seem that Delta's training is matching their policy now.

After reconnecting, the second rep pulled up my ticket, confirmed the upgrade (Z) space (though there were fewer seats available than ExpertFlyer found when I searched it just before buying the tickets). I made sure to inform him that I wanted to upgrade all segments and all upgrades to business (not premium economy), but he had already assumed that. He waited on the line while I re-checked the AF website to confirm that it showed the trip was all in Business class. The entire process with the second rep only took about 30 minutes, with most of that time taken up with him on hold with AF.

Based on the posts in this thread, I expected a much longer and more stressful process, but it went smoothly.

Any insight on expected MQDs when using GUCs? AF shows a 057 ticket in business, but Delta.com shows both the 057 ticket number and a 006 ticket number along wih a different PNR number.
Thanks again