Only reason why it has to be done on the phone, is that the extra comfort seat, does not include taxes.
That is why a human has to do it.
If you are WP, you could/would get the HBA call centre, otherwise, your call would most likely get routed to the overseas call center.
Not saying the overseas ones might not know what you are after, but they tend to probably/possible make more errors, than say the QFs own HBA staff.
Overseas call center, also the call quality might not be the best.
If you do not want to call, you could book online the premY cabin on the 787.
Then do OLCI, and pick to sit together, show up to show id asap for SYD, ie, 3 hrs prior, and get the orange sticker.
Dont loose the BP as you will need it when you get to PER.
Also, there is a small chance that QF might swap out the 787 to you being on a dom 737, SYD -> PER, if that 787 goes tech(no).