Seems like an error to use a URL. This is perfect for a URN or some other form of URI.

It could be a URN that used the DNS as a back end (though that’s a lot like a URL) so better would be something more abstract with multiple possible resolvers and a signature.

The only useful alternative you're talking about would be a content-addressed one. A "URN" that relies on some kind of mutable registry lookup is actually a URL (both are URIs).