Is it possible to use them? Specifically with where... Something like:

posts(where: {_and: [{categoryName: "foo"}, {categoryName: bar}]}) {
    edges {
      node {
        id
      }
    }
  }

If not, can anyone think of a workaround that achieves the same thing?

I'm thinking I could do this:

posts(where: {categoryName: "foo"}) {
    edges {
      node {
        id
        categories(where: {name: ["bar"]}) {
          edges {
            node {
              id
            }
          }
        }
      }
    }
  }

And then some operation on the returned data that filters out posts with falsy categories objects?

I can see a potential downside being we're fetching data we don't need.