I have had success on one occasion SDC'ing the return domestic portion of an Intl itinerary.

I was flying NRT-HNL (stopover) HNL-US and they let me SDC the HNL-US flight. I reconfirmed it with TWO different agents by phone - both said it was fine as there were no further Intl segments in the itinerary. So there you go, YMMV.

Ironically, they wouldn't let me do SDC on US-Canada recently because they said it was "International" even though it is clearly allowed in the rules as posted on Delta.com. When I advised that it clearly includes Canada on the website, I was told that the website is not binding and that the rules "in their book" supersede the website. I'm sure DOT and advertising standards would like to hear that. In any event, it became a big argument, and involved speaking to the assistant station manager.