Well after checking again on JAL website and seeing lots of flights open, I called in and was able to get a USM-BKK ticket. Took 30 mins on the phone but was otherwise painless.

They needed to "fill-in " the booking to ensure the ticket was there, so doesn't seem like they can just tell you what flights are available. Come prepared with flights first from Google Flights or JAL (though from my previous experience I wouldn't necessarily trust JAL).

Price was 5000+300B. Transfered AMEX MR instantly from my account after they had done the "fill-in" booking check.

BKK-USM is 100B tax. Flights were selling for $105 on Google Flights (and via Ultimate Rewards).