Composer: Composer repository for dynamic packages receives no param for searched package?

I am in the process of building a _Composer proxy service/ repository_. Idea of the mini-service is to dynamically deliver language/ translation packages for themes or plugins from _WordPress_ API responses. The routes are the following:

 https://api.wordpress.org/translations/plugins/1.0/?slug={slug}&version={version}

The _Nodejs_-service itself is nothing than a proxy that should be added as repository to a composer.json of type:project. The service itself returns the providers-url-key as part of the (dynamically generated) packages.json file.

"providers-url" : "/" + request.params.scope + "/providers/%package%$%hash%.json"

For an example, please see the following composer.json:

{
    "name"              : "vendor/testproject",
    "type"              : "wordpress-project",
    "minimum-stability" : "dev",
    "repositories"      : [
        {
            "type" : "composer",
            "url"  : "https://{unique-ID}.ngrok.io/test"
        }
    ]
}

So far I can only see requests to /endpoint/packages.json, but without any params. In an ideal world Composer would send along the required packages, either as header or as query param.

I already looked (and tested) through all available options to try to _fake_ my way through the system to somehow get the requested package: type:{composer|vcs|…}. Is there a way to make this happen? Or is there at least the _will_ to accept a PR to implement this – alongside an additional flag to enable that behavior?

_A note regarding hosted services: there could be a combination of notify-batch and request/require options to gather some stats about searched and actually fetched repositories. This approach might allow to target "to-cache" packages much better._