One of the things I love about Guix is how easy it is to modify 
dependencies, a feature I use fairly often on individual packages 
or manifests. However, I'm struggling to get a similar effect 
system-wide.

Grafts do exactly what I want, but since they are defined in the 
upstream package definition I can't set them for my personal 
system. Package rewriting lets me do this on a plain list of 
packages, but it's really difficult to do the same for services 
and not possible for guix shell/other command line usage.
#+begin_src scheme
(operating-system
 ;; Package rewriting makes this fairly straight-forward
 (packages (fix-pkg %my-packages))

 ;; But for packages deep in the dependency tree, you have to
 ;; track down every service that has this package as a transient
 ;; dependency and fix it.
 (services
  (service some-pkg-service-type
           (some-pkg-service-configuration
            (some-pkg-service (fix-pkg pkg))))

  ;; Repeat ad-nauseum
  ))
#+end_src

If you're attempting to customize, say, Xorg, this results in a 
very-not-fun-time.

My ideal solution is something like the following:
#+begin_src scheme
;; When installed in a profile, replaces pkg for everything in the
;; profile
(fixed-pkg
 (replaces pkg)
 ...)
#+end_src

Or if that's infeasible, at least something like this:
#+begin_src scheme
(list (channel
        (name 'guix)
        (url "https://git.savannah.gnu.org/git/guix.git")
        (replacements `((,pkg . ,fixed-pkg)))
        ...))
#+end_src

I'm thinking this falls into one of the following:
1. I'm dumb and there's an obvious solution
2. I should be using a local checkout for this type of work and 
not the upstream Guix channel
3. There's no good mechanism for this, and a patch would be 
welcomed

I'd appreciate any advice here, as this feels so much like 
something Guix should do surely others have stumbled on this 
before. Thank you!