I don't know about this. I just don't think Elvis would have done a duet. Would the Colonel even allow it? I doubt that this would have made Elvis excited about recording again. I can't picture Elvis sitting around at Graceland listening to Loretta Lynn.
Elvis was such a nice guy that he probably just didn't want to hurt her feelings. It would be nice to ask the MM if he talked to her on the phone. As everyone here knows Elvis didn't answer the phone or make a call without someone dialing the number for him. So they would know I'm sure.