to reply to myself, it appears it was a one-off sort of thing... I couldn't get the flight originally listed, but when I moved it to a different JQ flight the UA folks were able to issue the ticket.
The flights appeared identically in ITA, but I *think* what happened is that the flight either sold out or otherwise was schedule-changed or something, because now I can't see it at all.
Observation: when booking a JQ codeshare the UA agent/system has to wait 24 hours for confirmation from JetStar -- they held the reservation and fare, but I did have to call back the next day to get the ticket issued...