The situation is much better than you think. I actually have a little DSL project along these lines: it's meant to be a declarative component in a larger application which enforces certain data guarantees.

It is not based on JSON or any other out-of-the-box format. The only source material for this DSL anywhere in the world is on my laptop: a little documentation, a few examples, a partial implementation.

This is enough. Codex can not only give me arbitrary examples in this novel DSL on demand, it can see what I'm trying to do with the compiler and extend it for me.