Originally Posted by SNA_Flyer
Your best bet is AUS-LAX-HKG-BKK on Cathay, then BKK-REP on Bangkok Airways. You would need to purchase the BKK-REP-BKK, since Bangkok Airways does not participate in any mileage schemes (should be about $300pp RT). The next best option would be Bangkok via Tokyo on AA/JL.
Cathay Business Class is defineately the way to go. With the amount of miles you have, you could do First, but getting two First seats is going to be virtually impossible on CX.
I'm in total agreement with SNA_Flyer. It will be virtually impossible for you to get 2 F award seats on Cathay, but their J is very nice anyway. The itinerary mentioned will cost you 110,000 miles each for a J award, which is exactly what I did earlier this year.
Actually that's only 3 legs to get there, FYI, but YES, CX follows the same 330-day rules, and stopovers are definitely allowed. The nice thing is that the one super-long flight (LAX-HKG and back) is perfectly scheduled overnight in both directions, so you can get TONS of sleep. I would recommend doing a stopover in HKG in both directions for a day or two, and of course spend a couple of days in BKK on either end of your Cambodia trip. Although the flight to Siem Reap is less than an hour, so that won't bother you too much.
HKG and BKK are GREAT cities with tons to do and see. If you really want to treat yourself, the Shangri-La and Peninsula hotel chains have wonderful offerings in both cities, and the Mandarin Oriental is really nice in Bangkok also. The Chi Spa at the Shangri-La is FANTASTIC, and you'll definitely want to have tea at the Peninsula.
There is a day trip to Ayutthaya (the ancient capital of Thailand) which you can book directly from the Shangri-La Bangkok. It costs about $45, and you get to see a couple of old temple ruins, an active temple, the royal summer palace, and then take the Shangri-La's boat back down the river while you dine on a great Thai buffet.
As for getting to Cambodia, your best bet is going to be using Bangkok Airways. However, my advice based on my own experience is to book an actual tour through Pink Rose Holidays. They are a subsidiary of Bangkok Airways. Their website is listed here:
http://www.pinkroseholidays.com/pinkrose/
We did the 2-day/1-night tour, A Glimpse of Siem Reap. It cost us about $450 each and covered our flights, hotel, ALL food, and tours of all the temples. The best part was we had a car, driver, and guide all to ourselves. They picked us up from the airport, took all the tours at the pace WE were comfortable with, and we even got to see traditional Thai dancing at dinner. You may want to take one of the longer tours if you're REALLY into the Khmer history, but this one was perfect for us. Angkor Wat is truly spectacular, and Ta Prohm was a beautiful, haunting surprise.
Avoid the hot and rainy seasons by going over between October and March. We went late February/early March and the weather was perfect.