Comment 17 for bug 1890803

Possibly interesting is the fact that open-iscsi tests use overlayroot, and they are reportedly working:

http://autopkgtest.ubuntu.com/packages/open-iscsi/groovy/amd64

there is some more info on how those tests run at
https://git.launchpad.net/ubuntu/+source/open-iscsi/tree/debian/tests/README-boot-test.md