So, I am told by both my local office and Bangkok that a mixed-cabin 50% Award is not possible under this unpublished 'benefit'. Nor is a one-way 50% Award possible.
These would be possible using the 'new' awards system, just not for the 50% Gold/Plat Awards. I was told that the flights must be in the same cabin for both sectors, although the Auckland office said mixed award was available but at biz return rates.
Clearly, the rules for these are more restrictive than the 'new' award charts that do allow mixed cabin awards. It' s just a pity that THAI doesn't provide the details (at least at present).
Still, I gotta travel and there are no biz award seats for my return, so economy it is BKK-ICN-BKK. The economy award at THB 3700 is way cheaper than buying a Y ticket (THB 19,700 !).