Several (maybe 8?) years ago, on NW 001 (MSP-LAX-NRT-BKK--used to be all one flight number!), I was denied the Plat UG on MSP-LAX even with empty seats. They tried every which way but were denied--because it's one flight number, one coupon--even with a change of gauge at LAX and NRT. It was 757/747/747 back then.
Funny enough, NRT-BKK was oversold, and I ended up with an Op-Up--so, even with only one "coupon", I was upgraded on a different portion--and a much better (and longer) one!

I think that NWA only "solved" this problem by eliminating these flights with domestic and int'l portions. I don't think they have any now, though of course I could be wrong.