AFAIK, you are allowed one stopover (so to stay in a city more than 24h) and you are free to use it in BKK and spend some days there. And the good news is that you will get charged zone 2 to zone 8...
I think it's quite common by flyertalkers here to book a trip to zone 8 and making a stopover in BKK or SIN, isn't it (at least for leisure travel)?