However, before this is possible, we must first change all patterns like:

  (define-public python-netaddr …)

  (define-public python2-netaddr
    (package-with-python2 python-netaddr))

to:

  (define-public python-netaddr
    (package
      ;; …
      (properties `((python2-variant . ,(delay python2-netaddr))))))

  (define-public python2-netaddr
    (package-with-python2 (strip-python2-variant python-netaddr)))

and make sure (with “guix build python2-netaddr -d” and similar) that
the changes have no impact on the resulting derivation.

Volunteers?  :-)

Thanks,
Ludo’.