Hello Guix! Commit cd436bf05a8344acf4462f3602e7d360821a902a allows source tarballs to be fetched from content-addressed mirrors, such as tarballs.nixos.org (I expect other mirrors will be added soon.)
By default, URLs of the style http://tarballs.nixos.org/sha256/XYZ are automatically appended to the list of URLs that’s explicitly specified. Ludo’.