LOL same flight as me when I tried CX J; all the way to MEL I assume?
In which case, I wouldn't worry
too much about seat selection. You will get dinner service (express service available) then lights out fairly quickly. The crew aren't overly sociable and will be in the galley merely to respond to your requests (I just walked up; didn't use call bell).
As usual, avoid bulkheads due to the traffic to bathrooms, they are located on the K side, not the A side.
There's one reason to choose middle of the cabin - furthest away from the bathrooms (and your pax on either side will hopefully only go the the bathroom nearest to them, ensuring that they will almost always never cross your path).
The FT CX FAQ gives no specific information about good seats in J, although it does recommend not sitting in the second J cabin, which is although smaller than the first cabin, it is closer to pleb class and the proximity to the frontmost pleb toilets and the second J toilets + galley can mean more noise.
The champagne on serve was Deutz - not the greatest drop in the world by two miles, even when mixed into one of Cathay's signature drinks, Pacific Sunrise (champagne with Drambuie, add lemon & orange zest). Cathay's signature non-alcoholic drink - Cathay Delight (a green concoction) - is a very nice combination of kiwi fruit, coconut juice and mint - I'd highly recommend trying it.
Oh, and bring something to do in ADL. You'll be sequestered into the International gate lounges after you deplane in ADL and go through transit security. There is next to nothing to do there; there are no power points or desks (unlike in the rest of ADL airport); you will have no access to the QP; you can't go back landside. I listened to my iPod but otherwise I was bored absolutely ****less.
ADL-MEL refreshment service is a sandwich, a canape and a tiny salad with drinks.