An excellent example, and the only such route that isn't restricted on the OWE fare (round-trip transcons are prohibited in US and Australia, DXB etc. long-hauls are restricted out of LHR, etc.). I cannot think of any other route where extra segments are so useful and such a bargain! The other advantage of purchasing extra segments is that they are fully changeable for date/routing, unlike most discount tickets that might be cheaper; but generally buying separate tickets is more cost effective.

Why would JL joining affect the OWE price? JL adds very few new routes and cities (except within Japan and China). More likely are rule changes (such as limiting HKG-NRT flights similarly to how LHR is limited). Oneworld has been coy about exactly when JL joins, but there have been some press releases which imply that JL will fully participate in OW when joining, which could be interpreted to be part of the OWE fare from day 1. We'll see soon enough, I suppose. The one sure thing with OWE fares is that the rules and fares change every year, but I don't see this year having bigger changes than in the past.