For example, 

edit-release-group.url.0.text=http://www.example.com/&edit-release-group.url.0.link_type_id=1

Does not seed external links.
This seems to work fine for artist edits and release edit (e.g., edit-artist.url.0.text=foobar), and also works for other kind of release-group edits, e.g., name, but the combination of release-group and url doesn't work.