Decode custom scalars from when creating mocks from json

is it possible to instantiate objects from local json (as described here) and yet leverage the custom scalars ?

I have a custom type public typealias Timestamptz = Date and a custom implementation

extension Date: JSONDecodable, JSONEncodable {
    public init(_jsonValue value: JSONValue) throws {

where I parse a string into a date. This works as expected when talking to the real graphql server.

However, when I init my object directly using a local json using

let jsonObject = loadJson() // JSONObject
let myObject = MyQuery.Data(data: DataDict(jsonObject, variables: nil))

then access the date property myObject.created_at

I get the exception

Could not cast value of type 'Swift.AnyHashable' (0x10834c1a0) to 'Foundation.Date' (0x106ec74d0).

because the underlying value is indeed still a String and not a Date

Looking at the sources, I see that this mapping to the custom scalar init is done in the GraphQLSelectionSetMapper.swift#L10-L13

So the question is, how can I parse an graphql local json response into a graphql object for testing purposes?

I know that I could use the new generated Mocks but for my usecase, using the json reponses is more convenient.

thanks for your help