MEL - PEK on the A330 is a perfectly fine flight. I flew home from SYD in 2014 on SYD - PEK - NRT (both CA A330s) - IAD (NH). While it's certainly no SQ or NH it's perfectly acceptable for what amounts to a long redeye flight. They (CA) use the same J seats as TK does on the A330 which are perfect for sleeping.

When it comes to award tickets, A bird in the hand is better than two in the bush unless you are someone who is okay with gambling and making a last minute booking (the best availability for OZ/US flights is either at T-331 or at T-14 and less and T-331 has passed). Plus you can change the routing (for a relatively small fee) when you get closer to departure if/when better options open up.

Also on a UA award ticket you can have a layover of 23:59 (hours:minutes) so you could try and do something like:

MEL - PEK (overnight layover so you can poke around PEK a bit) - XYZ - USA.