This bug is linked from a reddit discussion at
https://www.reddit.com/r/GUIX/comments/9un20j/question_about_guix_pull
mentioning ludo's solution.

If anybody finds this needing it like I did, I've tried to upgrade his code
to work on a modern guix at
https://github.com/nomr72/guix-substitutes-channel .  Changes in guile-json
stopped the old code from working.

I'm very new to guile, but it's working now.

Once the build infrastructure is developed enough to provide substitutes
very reliably, this will be unneeded.