Version 1.1.0 was released: https://github.com/coreos/rkt/releases/tag/v1.1.0
This is in git: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=127cb329540200ba43a973dd7d7b395596e1e0bb
I've added rkt_stage1_kvm support. Tested it with quay.io/coreos/alpine-sh and it worked, but I had to kill kvmtool manually at the end.