Hi Neil,
The App Engine
Search API service is accessible via the "API" of the libraries provided to the App Engine runtime in the given language. External access to the Search service for an app should be done through sending requests to handlers on your app (even an
Endpoints API) which can access the Search API. Therefore, writing a "client library" for a custom runtime would involve writing a "wrapper" or "middle-ware" application in Java, Python, or Go which could access the Search API.
I hope this helps you move forward with your development, and specifically in reference to custom runtimes, I'm sure development will proceed with making these services accessible via a REST API (don't quote me, though), rather than simply being embedded in the runtime for the main supported languages of traditional GAE.
If you'd like, there is a
public issue tracker for App Engine, and I encourage you to create a feature request asking for a REST API for Search API (and any other App Engine services which don't expose a REST API).
Sincerely,
Nick