How can I map a type to an external reference like with openapi-generator's typeMappings?

[quasi-coherent]

A colleague of mine needs to use this tool to generate a GraphQL schema from an OpenAPI specification of a server. I wrote this OpenAPI schema to also generate Rust server code, but for "technical" reasons, one type is actually declared like this

MySchema:
  type: object
  properties:
    id:
      type: string
    thing:
      type: ExternalThingSchema

The reason is that what the type of thing is, is actually a very complicated schema from a different OpenAPI spec, and again, for reasons, I don't want to use a $ref. But we actually generate Rust code from that already, so this is fine, because I can configure the generator with

typeMappings: 
  ExternalThingSchema: "my::generated::code::Thing"

and the generated code maps ExternalThingSchema to a valid reference to a type.

This schema is part of a response, but since this is not "valid" OpenAPI on its own, graphql-mesh obviously can't handle it. My question is this:

Assuming that there exists already a GraphQL schema that does define the Thing, is there some way to achieve a similar end? I've looked through the docs and argTypeMap is kind of along these lines, but not really, and obviously that's only for request types.

The documentation on responseSchema is sparse, but all the examples I've seen reference a file that contains the whole schema, which sort of defeats the purpose.

Maybe there's a way to generate a sort of generic placeholder type for MySchema (or ExternalThingSchema?), and then unify that with the one already defined that has the real type?

Or maybe start with the generic placeholder type again, and then use the replace-field transform?

transforms:
  - replace-field:
      typeDefs: |
        type ExternalThingSchema {
          thing: Any # Can this be a scalar?  Trying to avoid changing the openapi spec
        }
      replacements:
        - from:
            type: ExternalThingSchema
            field: thing
          to:
            type: ThingSchema
            field: thing

Maybe a simpler question that could help a lot: how can I get it to generate something, anything from such a schema? I feel like I'd be able to try a lot of things if I could do that.

[ardatan]

If it is not a valid OpenAPI spec, we don't have much to do with that.
You can use JSON Schema handler to define your schemas for each endpoint manually.

[quasi-coherent]

If it is not a valid OpenAPI spec, we don't have much to do with that.

It's a code generator, not an OpenAPI Specification verifier. Swagger, the OpenAPI Generator, and most other tools not affiliated with those projects support this. A parser runs over the OpenAPI. It maps types according to a configuration option. Or more generally it can do a lot of things based on configuration. It passes the output, representing a valid OpenAPI document, to a generator. It's surprising to me that this tool insists that no special pre-processing would ever need to be done.