I am flying IAD-SFO-ICN-PEK with IAD-ICN booked on UA as one PNR and ICN-PEK on OZ on a separate PNR.

I will only have a carry on so would love to be able to check in for the OZ leg in advance. The layover at ICN is from 4-8am so plenty of time if I need to enter immigration then check in.

OLCI seems to be disabled since there is a document check for mainland flights.

I see there as being 2 options to get the BP in advance:
1) trying to get UA to issue the BP at IAD
2) leaving security at SFO to go to the OZ desk (which will be open for their flight to ICN)

Any ideas for how to get this to work?