Just for anyone interested, I published an unofficial CoreOS release (hikey-coreos-8) that now works on the Hikey board. See
https://github.com/glevand/hikey-coreos/releases/tag/hikey-coreos-8
All the basic CoreOS services; cloud-init, etcd, fleet, flannel, are working on arm64 and allow for mixed architecture clusters, with some nodes arm64 and some nodes amd64, to be created. See https://coreos.com for more info on CoreOS.
To get this to work on Hikey I needed to hack up the CoreOS build system up to support a second kernel just for the Hikey. My stomach starts to feel sick when I think about it, but along with this I needed to add in a 90,321 line kernel patch from hikey-mainline-rebase. There is no way this stuff can be merged upstream.
-Geoff