One of the FAIR principles is that "(meta)data are released with a clear and accessible data
One way to express the usage rights of metadata in a machine readable way is in the structured metadata document of each dataset. I described how to do this in the version of DDI that Dataverse exports for published datasets. I'm sure it's easy to do in the "Dataverse_json" export. Some of the other metadata standards might not include machine readable ways to distinguish between the usage rights of the metadata and data that it describes.
Or maybe there's a way to include usage rights of the metadata exports in a machine readable document that describes the repository as a whole?
Last, I should say that I don't know of any Dataverse repository that needs to restrict the use of its metadata, and the principles of openness are so baked into its development that I can't see how the issue of metadata usage rights can ever be as contentious as it's been in other efforts to aggregate metadata to improve the discoverability of information.