I do exactly what you suggested:

--8<---------------cut here---------------start------------->8---
(defun ambrevar/init-guix ()
  (and buffer-file-name
       (string-match "\\<guix\\>" buffer-file-name) ; Adapt this to your install.
       (guix-devel-mode)))
(add-hook 'scheme-mode-hook 'ambrevar/init-guix)
--8<---------------cut here---------------end--------------->8---

A better approach might be to add it to .dir-locals.el upstream.  What do you
people think?

-- 
Pierre Neidhardt
https://ambrevar.xyz/