You may be able to use a kiosk in the international ticketing area of SFO and scan your passport then to receive your boarding passes. If that fails, you can approach the QR check-in counter, but they may not be manned until 3-ish hours prior to your departure. YMMV. I've been able to check in plenty early(10 hours prior to departure) via CX kiosk in LAX on a connection, but could not do the same on Qantas through SFO.

If you're checking baggage, you won't be able to check until the desk is manned by QR for your particular flight. So be prepared to haul your luggage around.

I ran into these logistics issues flying on an AS - Qantas itinerary through SFO. SEA-SFO-SYD. AS did not/could not issue a boarding pass for Qantas at my origin. Kiosks were available at SFO, but none would allow us to check in without seeing an actual person. We also had 7 hours to kill. Since we were stuck landside we just decided to go out for dinner and a drink, but had to drag our bags around with us (carry on size, but still not fun).