I just ticketed a DONE4 starting in Japan.
1) I am fairly sure that in order to get the Japan pricing, you must have it ticketed in Japan. Even if the itinerary starts in Japan, if you ticket in North America, it is priced as from North America.

2) The AA RTW desk were very helpful in setting up the itinerary, and were not bothered when I told them I would ticket it in Japan. They gave me a record locator and told me to call back in a day or two as the itinerary had to be sent to Dublin (?) for pricing. In fact they called me back the next day to point out a small problem.

3) Then I contacted the AA office in Japan by email, gave them the record locator, and they confirmed the pricing and I charged it to my credit card. They will not mail the ticket to the US (>16 segs so had to be paper ticket) but will mail it to a friend's address in Japan, who sent it on to me.
I was a bit shocked to find that it is not just a paper ticket, but five hand-written tickets taped together! I haven't had a hand-written ticket on a major airline for years.

In case you are interested (as your itinerary is similar to mine), mine is:
NRT-HKG-JFK-LAX-SJO-LAX-ORD-SJU-JFK-GRU-SCL-BOG-EZE-MAD-MOW-AMM-MCT-LHR-HKG-DEL-NRT for 62,354 miles and cost JPY849,130 including all taxes.
I booked the first 6 flights and left the rest open.

Happy Travels.