When I booked an AONE4 through AA I asked if it was possible to book BA codeshares on SYD-CHC-SYD. I was told I'd have to book through BA to do that.

However, I was flying SIN-SYD-CHC and at the SIN lounge was able to get the SYD-CHC flight re-booked as the BA codeshare after I'd checked in. Not sure how they did it because the people in the CHC (ticketing and in the lounges) didn't know how the people in SIN did it!