Do you need to go via HKG? Using AA for the North Pacific crossing via NRT is generally the shortest way for a xCIRxx fare.

Consider BOS-ORD-NRT-BNE-LAX-BOS for the Circle Pacific route (21,335 according to Great Circle Mapper), and purchase a separate el-cheapo net fare for BNE-SYD-BNE, which is only a 1 hour flight and often go for under A$100 each way. That's got to be cheaper than upgrading to the 26,000 mile fare.

Or, if travelling in J, I would consider BOS-JFK-NRT-SYD-JFK-BOS (21,914 according to Great Circle Mapper) since that assures you of getting the QF Skybed seats all the way from SYD-JFK (via LAX on QF107), and buy your cheap SYD-BNE-SYD return flight separately.