I am on flights 27 (ATL-HKG) and 26 (HKG-ATL) couple of times this month with a layover and/or equipment change. I know, I should have booked them as separate legs but I couldn't as time was running out. Technically, I am on eligible for upgrade on ATL-DTW and DTW-ATL segments. Is there a work around in order to get an upgrade on the domestic segment.

When something like this occurred on NW, the gate agent usually worked something out. The GA uin ASTL gave me the official line.

I know DL says you can't get upgraded on domestic legs of through international flights if you are booked as a through passenger. This is rather silly. A more frequent and higher status passsenger is denied an upgrade!.
I'll look up the Medallion rules. As far as I can recall, there is nothing that says that you are ineligible for upgrades on domestic sector on purchased ticket, under some conditions. If DL does bot explicitly state that you are ineligible for upgrades on domestic segments of international flights, I believe DL is breaking its promise for eligibility ffor upgrades on domestic flights.