Scratch my previous answer - it's undocumented, but it IS possible, I found it by reading the compiler code. Bah.

go.tag

This piece of thrift code:

struct foo {
  1: string bar (go.tag = "json:\"baz\" yo:\"dawg\""),
  2: string bang
}

Generates the following Go code:

type Foo struct {
        Bar  string `thrift:"bar,1" json:"baz" yo:"dawg"`
        Bang string `thrift:"bang,2" json:"bang"`
}