MIME media type (content type) for representing a URL?

You should be able to use text/uri-list defined in RFC 2483 Section 5. You are only returning a single URL, but there is nothing in the spec saying it can't be a list of one. To conform to the spec, just make sure the URL is all on one line and terminated with a CRLF pair.

text/x-uri should be fine, it's the most used as far as I know.