Why not have the dataset name as a fragment in the URL? For instance:

    http://demo.noms.io/cli-tour#sf-fire-inspections/raw

Glancing over RFC3986 [1], fragment identifiers seem to be pretty much made for what you're trying to communicate with :: - separating a subresource (the dataset) from a primary resource (the database). Unless I'm misunderstanding something?

[1]: https://tools.ietf.org/html/rfc3986#section-3.5