I've got enough miles in my DL account and still have some PMU's from this year. Of course, they'll expire before the trip I'm trying to book, but I've already qualified for PM next year and therefore have SWU's coming in January-February, before the trip.

I gave it one more try yesterday. Called the DL Plat SMS number, punched in my SkyMiles number. Somehow (wonder how?) I wound up with an excellent agent. She found M fares now displayed on DL.com because it was a combination of DL and NW flight numbers. She found upgradeable inventory on all of the long-haul legs and one of the connections to the gateway. Fare was acceptable. Pulled the trigger. But then she went to process the upgrade, and that's where the snafu was. She could confirm the upgrade on the outbound but not on the return, even though the U inventory on NW was showing available. She spoke to several different agents trying to confirm it, but just couldn't make it happen. The only explanation she was given, which she didn't consider satisfactory, was that maybe the availability wasn't loaded into the system yet, and maybe it would be by next January or so. Regretably, I had to cancel it all out.

But it seems that this agent, whom I assured had done better than anyone else I'd spoken to, wasn't troubled by the lack of SWU certificate numbers. Must be just an issue of getting the right agent.

Still, it shouldn't be this hard to spend money with Delta.

Thanks to the responders who gave me helpful tips here.