Nope, it seems that SFO->NRT->SFO plus the consolidator ticket NRT->PVG->NRT is running cheaper than the ticket to PVG with a stopover in NRT since the first can be booked in S, while the lowest fare allowing the stopover seems to be H. There is also an extra $100 charge for the stopover, at least on UA.

At least that's the case for my next trip, which is planned for end May/early June (i.e., the expensive season). Might not be applicable for April, I haven't checked.