Can a user defined keyword modify the validated data?

[segevfiner]

I want to add a keyword that allows overriding values based on environment variable names that are defined in the schema, e.g.

{
  "type": "object",
  "properties": {
    "foo": {
      "type": "string",
      "x-env": "MY_FOO"
    }
  }
}

But I'm not sure if that is possible or how to achieve this. It seems that validate based used defined keywords won't work with standalone code generation? So I'd rather use codegen, but can a codegen keyword even modify the data?