Pierre Neidhardt <ambrevar@gmail.com> writes:

> Is there a good reason not to allow local paths in git URLs?

You might find the "local-file" procedure to be helpful here.  You can
pass a local-file as the source of a package, which should allow you to
define packages that use a local file path as the source.  See (guix)
G-Expressions in the manual for details.

-- 
Chris