So I called back today. I had a schedule change on an itinerary in November, so I got that confirmed and asked about the upgrade on the LIH - SFO flight.

The agent put me on hold and I waited for about 10 minutes while she called somewhere else. She came back and said I've now been added to the 'priority' upgrade waitlist. Log out and back into united.com and I can now see the waitlisted upgrade for the flight.