Greg,
As a Family and Friends traveller (aka buddy pass) you are only eligible to fly on Delta operated flights. That unfortunately means that BKK is out of the question. Employees and their spouse and children can purchase what the industry calls ID9x tickets on other airlines, but Delta's buddy pass program only allows for travel on delta flights. (Codeshare flights are not eligible) As far as yield fares go, you can only get that on TravelLine or online on DeltaNet (you need the employee for either though).

Hope this helps.