I agree, I think it would be useful.
Maybe to be able to define a custom stream, would be great as well.
My idea would be something like this: coreos.inst.stream={url} -> url could be:
http(s):// or file://
The upfollowing updates should be gatherd from this defined stream as well. This would
require to be able to manipulate and change the ostree of fedora coreos before it has been
composed.
Basically:
https://pagure.io/fedora-atomic for fedora coreos. (fedora-atomic-host.json
-> fedora-coreos-host.json?)