Pure conjecture on my part, but perhaps getting the originating airline to issue the boarding pass for the next ticket should take care of things?
Have done that CX->AA, CX->LA, AA->CX and know of people who have done UL->CX, MH->UL, MH->JL and CX->JL

(and I did have an unsuccessful attempt at LA->CX, but that was because I was checking in on a third LA ticket, for a LA->LA->CX on 3 different tickets, and they could only manage LA->LA on separate tickets..and the check in agent/supervisor had been pretty grouchy)