(for whatever reason, looks like this didn't end up in my inbox)
> +@item @code{initrd-extra-module-paths} (default: @code{'()})
> [...]
> The items in this may be either file-like objects (usually 
> packages), or
> +a list where the first element is a package and the second is an
> +output--e.g. @code{(list (list zfs "module"))}.


As-is, I don't think this is a good example, because
'expression->initrd' does not set #:substitutable? #false,
the 'zfs' package has the comment (*)

     `(;; The ZFS kernel module should not be downloaded since the
license
       ;; terms don't allow for distributing it, only building it
locally.
       #:substitutable? #f [...])

and IIUC, the code inside expression->initrd copies the kernel module
into
the new store item, so it looks like this accidentally suggest people
to
commit copyvios, and copyvios are currently against the law.

Greetings,
Maxime.

(*) Though I don't understand that comment: Guix is a distribution, so
by definition
it's distributing zfs -- unless it's only talking about the binaries
and not the
source code ...