Thanks ernestnywang for those investigations / demonstrations. I think the sooner that ExpertFlyer adds a POC option the easier it will be to at least understand/plan around this particular challenge.

Particularly in JAL's case there is clearly something else going on as well beyond POC - as most of my RTW tickets are ex-TYO, but you still can't see space on most JAL flights in A or D, though it would show on the JAL website even for a direct sale - but hey, it is appreciated that you have experimented.