Mmm, interesting.

Well you would need to check in for both sectors and get a boarding pass off the interwebby or one those box things. Then the critical bit is you must be scanned at the gate for the WT+ sector or you'd lose the return. The trouble is that I can't see a gate agent being prepared to also scan your CW boarding pass as well, without some slightly underhand jikkery-pokery at the gate. And if you don't scan your CW boarding pass then a standby - typically a staff member - would get your CW seat. Not sure how you could get through Ready to Fly conformance either, I see even less flexibility with HAL staff. Pray for an Airport Upgrade Promotion?

In the reverse direction it would have been easier, obviously.

Not sure that helps much!