Comment 12 for bug 1414466

That's rather embarrassing, but the problem with my VM was that it was lacking networking support. I turned this (too) minimal example of a Guix System:

;;; file: os.scm
(use-modules (gnu services ssh)
             (gnu system)
             (gnu tests))

(simple-operating-system
 (service openssh-service-type
          (openssh-configuration
           (permit-root-login #t)
           (allow-empty-passwords? #t)
           (log-level 'debug))))

Into:
;;; file: os.scm
(use-modules (gnu services networking)
             (gnu services ssh)
             (gnu system)
             (gnu tests))

(simple-operating-system
 (service dhcp-client-service-type)
 (service openssh-service-type
          (openssh-configuration
           (permit-root-login #t)
           (allow-empty-passwords? #t)
           (log-level 'debug))))

After which using the '-nic user,hostfwd=tcp::3333-:22' allowed me to SSH to localhost port 3333 successfully. Closing!