If you will notice when Hawkeye is taking Hojon to the induction center in the Korean town, you will see many people wearing the pointed type straw hat for shade from the sun. These hats are particular for the Vietnamese and Thai peoples - NOT SEEN IN KOREA!
Also check the name Hojon. That is not a Korean name.
The vietmanees references to culture and names were done on purpose. MASH is an anti war movie. They set it in Korea because the studios wouldn't accept it set in Viet Nam. They had a really good interview with the writers and directors on the history channel last spring. It also explaned about no black surgeons in Korea as well.
Originally, there was no mention of Korea in the film at all. Altman designed the film to be like the Vietnam War, which was raging in 1969 and 1970 (when the film was in creation and release). The studio forced Altman to add the specification of the Korean War at the beginning of the film to avoid controversy (a highly ineffective ploy!).