Ludovic posted some channels.scm code in September 2018 supporting using more substitutes. Unfortunately his code no longer functions due to an upgrade of guile-json in guix.
I've tried to learn enough guile to upgrade the code to the newer json structures, and posted my changes to github at https://github.com/nomr72/guix-substitutes-channel . It seems to run again now. Maybe I can learn enough to upgrade it to check the 'guix-master' evaluations to find the latest build of key packages. We'll see.
