How would one create an identifier template?

[Sidetrack]

I've got some books from archive.org and I've got urls as identifiers, but I'd really like to create my own identifier type with a link and display template

I've got

url:http://www.archive.org/details/alicesadventures19033gut displays as www.archive.org

but I'd rather have

archive.org:alicesadventures19033gut displayed as archive.org and still linked properly with it's own selectable identifier category in the tag browser

I can't find a way to set up a template though short of, I assume, creating and metadata import plugin. Am I missing some method inside of Calibre to do it?