We eventually do want to get metadata support fully (types and all) for Columns especially and perhaps Records etc. in the future.https://github.com/OpenRefine/OpenRefine/issues/1726
and probably a few others that are open with different requirements/use cases.
We have the specific label:metadata that you can see those issues/enhancement requests.
And the broader query of just "metadata"
The other place where metadata is supported is on a Project itself (name, description, tags, etc.) which I'm sure you saw with it's basic Key:Value layout.
I'd love for you to work with Antonin later on in September when he has time to possibly collaborate on the first steps of metadata support again for OpenRefine. That would be fantastic! And your use case certainly fits into the basic criteria.