>> Does this approach (putting project root directories into
>> file-name-handler-alist) sound like something that could be accepted
>> into core Emacs? If so I'll get started.

OK, I implemented a very rough prototype of this. It is
attached. environment.el is the API that would go into Emacs,
guix-environment.el provides a command to use an guix-provided
environment, and test-environment.el is just a sequence of expressions
to evaluate to demonstrate them. (Not tests)