This is likely too restrictive - I've previously argued that what you really want is the contextual type & Record<string, any>.

Isn't Record<string, any> equivalent to { [s: string]: any }? JS objects include a string indexer now, so this is legal:

const o = {
    p: 12,
    m() {
        return this.p + this.x
    }
}

And o: { p: number; m(): any; [x: string]: any }. Does that address the scenario you had in mind?

From conversation with @Andy-MS, note that if you make a call like this:

transformsPropertiesIntoMethods({
  p: 12,
  m() {
    return this.p()
  }
})

Then you will get an error with // @ts-check because the contextual type comes from the object literal but should come from the call to transformsPropertiesIntoMethods. You'll need to install types for the transformer function (ember or vue are popular libraries that work like this).