Bringing it slightly back to the OP, for starters UA and NH Premium Economy are two completly different products. UA they just stretched out the seat pitch, same seats, same everything else, just stretched out a bit. NH they have a separate cabin, improved seats, separate check-in, separate, improved everything (except F&B). You can't think of it in the same sense as the UA product, because it might have the same name, but is definetly not the same. To book confirm their seats, you have to pay the appropriate class, even NH Diamond/Platinum members can't reserve in advance or select, only get them on first-come basis at check-in for remaining seats.

As per your status, UA *S on an NH flight, would only put you on the op-up list, at the bottom of it, if the flight was oversold. Their ranking for op-up is as follows:

NH Diamond (*G)
NH Platinum (*G)
non-NH *G members
NH Bronze (*S)
non-NH *S
NH reg. member
other *A member
not registered

The bottom 3 there wouldn't even make a list, but if the REALLY needed to would stretch that far to upgrading a club member versus a non-member. rare, but has at some point most likely happened.

That being said, its better to be on the list, even if at the bottom than those who have no elite status at all, or not even enrolled in a program. I have seen situations where a NH *S was upgraded at check-in to PE, as well as non-NH *G, but non-NH*S I not too confident or sure that would happen. If you were the only *A elite member on the whole flight in oversold Y than possible, anything's technically possible. BUT I wouldn't be getting my hopes up on that one.

Good Luck!