The simple answer is: it depends on how Qantas configures your 744 (in the case of the A380 the answer is no, never.) Currently Y bookings are heavy and J bookings are light across the pacific, so Qantas is opening up the last few rows of PE for Y pax and the last rows of J seats for PE pax on some flights. If they do that then, as an Emerald, you can reserve them (just check the seat map on expedia first to see if the first few rows are 2x4x2 -- that means PE seats.)

However that can always change, and if your final destination is SJC then I have a better idea. Here's a trick for ya. The old 2-class 744s don't have premium economy seats installed yet. On those sectors, PE sits in Skybeds. No ifs or buts, no tricky risky seat selection required. One such route is SYD-SFO, which is only a half-hour drive from SJC. I'd book a PE ticket to SFO (pay for discount Y and use points to upgrade?) If you must go via LAX then alternatively you could fly via AKL, coz the AKL-LAX sector is also 2-class with PE in Skybeds.

That way nothing's left to chance and you get a business class seat for a fraction of the price.