QF domestic metal, BA codeshare, possible to hop on an earlier flight?

[pilot]

Our corporate travel agent has booked me CBR-SYD-LHR on BA, with CBR-SYD obviously on QF metal with a BA codeshare. The connection in SYD is just under 2 hours, which I'm not happy with. I would like longer so that I can have a meal and relax in the Flounge prior to the long flight in whY.

Problem is, there is only one BA codeshare on the day, so I'm stuck with it.

If I turn up at CBR early, what are my chances of getting on an earlier flight? I'm interested in:
1. Whether it is allowed (I suspect it isn't)
2. Whether it is possible with a kind-hearted check-in agent
3. If anyone has any experience of doing (or even trying) this.

Thanks in advance.