Here's a more complete proof of concept:

The attached file guix.scm, when run a checkout of emacs branch emacs-29
with guix build -f guix.scm will :
- compile everything as if run locally but with the patches provided by
guix sources.
- a local edit and the rerun of guix build -f guix.scm will only compile
  new / changed code. 

Now I can try developping emacs without worring about huge compilation
times and without the developping issues that would happen if I ran the
build-system by hand without the patches. 

We could even imagine adding an option to guix shell -D that could drop
such a file for any package, since most of the code is reproducible
(although applying a derivation this way is still quite hacky).