This definitely applies at DFW, so OP’s return leg is easy. I believe it does at LAX too, but am not 100% sure on that one.

The reason it won’t price is because BA has no interline fares filed west of the US into Australia. They can only plate an AA fare if it forms part of their joint business with AA - and Australia is definitely outside of scope here (and they’re strict westbound, because AA has similar partnerships with JL and QF that BA aren’t part of). Their own relationship with (big shareholder) QR can’t help matters either.

It’s a far cry from the days when LHR-LAX-AKL (the second leg on QF) was the default option for getting to NZ. A combination of broken relationships and increased US bureaucracy on connections put paid to that.

That all said, if customers start booking westbound connections in big volumes (UA also offer a fair few through fares), they’ll inevitably end up looking at it again. They’re like sheep, you see.