The prospect of writing packages for everything that isn’t libre doesn’t seem very appealing. They should have a repo with libre stuff and another with whatever doesn’t for their definition.

This exists already - the nonguix channel on gitlab. You can also easily distribute your own channel with a git repo of package definitions. Say you want to publish a channel that is specially geared and curated to left-handed people, you can do that, much in the way as you can publish an Ubuntu PPA, and it will integrate seamlessly into Guix.