How to define a block that compiles to custom JavaScript?

I can define blocks like this:

namespace pxsim.myblocks {
  //% block="for $item in $list"
  //% draggableParameters
  export function forEach(list: any[], handler: (item: any) => void): void {
    for (let _item of list) {
      handler(_item);
    }
  }
}

This block transpiles to

myblocks.forEach(null, function (item) {

})

How can I create a custom block that transpiles to

for (let item of list) {
}

Start from the shown pxsim.myblocks namespace, the //% block annotation, and the generated myblocks.forEach output. Read the PXT block-definition and transpilation entry points to determine whether custom JavaScript output is supported and how it is configured. Done means documenting the supported approach, or clearly stating the limitation and any required constraints.