Al B and RichardMEL, thanks for the detailed responses. I originally had the idea because it's often very difficult getting QF's red e-deals for flights MEL-SYD on a Sunday after a weekend down in Melbourne, so I thought I could start my RTW a little early. I gave up on the idea, partly because when I try to be a bit too clever it usually ends up biting me on the arse, and partly because I'd have to use UA (assuming they agreed to it) and I wouldn't want my RTW issued on UA stock. I'd hoped that TG flew MEL-SYD as well as SYD-MEL but sadly, no. Besides, I don't think I have the spare mileage on my ticket to allow it.
Anyway, it was an interesting thought while it lasted, and planning this RTW has been a great learning experience!