Another way might be to create a program that checks all the source repos of all the packages. When a new package source file with a higher version number gets uploaded, guix automatically builds the package and uploads it.
With the number of packages available using Guix (and it seems to keep
increasing) keeping up on the latest upstream versions manually is
gonna be a pain. :) Shouldn't the checking be automated in some way?
One idea might be to create a bug in the GNU Bug Tracker when a new
upstream release is detected.