What works:

  nmcli c add con-name dum0 ifname dum0 type dummy \
      tc.qdiscs 'ingress, root pfifo_fast'
  nmcli c modify dum0 -tc.qdiscs 'root pfifo_fast'
  nmcli c modify dum0 +tc.qdiscs 'root handle 666: fq_codel'