Well, finally got a chance to try this, and they DID interline, although I think it was only due to a good check-in agent.

I was traveling CFS-SYD on QF, and then SYD-SFO on UA (in Business, although I doubt that matters). Both were award tickets, and thus had to be booked separately. Whilst talking to Qantas the day before on a different issue I asked if they could link the two itineraries and they said no - but did offer to add a comment with the UA flight number on the QF booking.

At check-in I asked if they could interline the bags, and I was told that the rules had changed, but she would try. She asked for the ticket number on UA (not just the confirmation number) which thankfully I did have, and then did some typing and said that they "normally can't do it", but that she would "see what it would allow".

After a fair amount of typing out spits a baggage tag with SFO on it, and she the bags were checked all the way through! On principal I even walked from T2 to T3 and re-cleared security just so that I could catch the QF bus to T1. Catching the T-bus would have been easier, but just didn't seem right

Definitely seems like it was an exception, and I'm not sure if having the UA flight listed in advance mattered, but all up a good result.