You are technically eligible for upgrades on codeshare flights operated by DL, but in practice it doesn't really work. You don't get added to the list until you check-in, or in some cases until you ask to be added at the gate (at which point many seats are long gone). And in my experience the priorty doesn't seem normal (I seem a lot lower than I usually am). That's anecdotal of course.

If anyone that's received an comp upgrade on a codeshare can speak up, it would be interesting to hear. But in general, I just assume it's not going to happen and don't bother trying anymore.