I don't really know how the D fare class works. My experience is usually on advance purchase fares. All I can offer is my experience getting UA to do what they say they never will. In practice, they will sometimes accept unendorsed tickets from other airlines. But it's something that happens at the ticket counter or gate.

I have had some mixed success in approaching the UA ticket counter with a paper CO ticket. Sometimes the UA agents will take it. Other times they say no. If it's a restricted fare, the official word is that the ticket needs to be endorsed by CO to UA. The people at CO are not likely to do that for you. Don't despair. Approach UA anyway. They can indeed take it, even if they say that they can't. Try it. The worst thing they can say is no.

When it worked for me, I think it depended a lot on my status and personal favor with the UA agent accepting the ticket. I'm told that it creates some hassle in the accounting department. You didn't mention your status, but I think if you are 1P or 1K you stand a much better chance.

It's good that you are thinking of just doing this on the return portion. If for some reason you talked a UA agent into accepting the outbound CO ticket, you would probably have the remainder of the reservation cancelled by CO because you were a "no-show." And then you'ld likely be stuck in GSO without a res.

Your post implies that you have a paper ticket. That's to your benefit. This is one of the reasons I always try to use paper tickets. In a tightly timed situation, it can be tough to get e-tickets printed on paper.