You don't need to fork the entire OS, though - just provide the relevant non-free packages on top of the regular OS.
Guix devs themselves have hinted that "adding impurity to a free OS is much easier than removing impurity from a non-free OS", so I suspect they may not be categorically opposed to the idea of non-free packages.
Guix devs themselves have hinted that "adding impurity to a free OS is much easier than removing impurity from a non-free OS", so I suspect they may not be categorically opposed to the idea of non-free packages.